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New high temperature gas flow cell developed atISISTo cite this article R Haynes et al 2010 J Phys Conf Ser 251 012090

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New high temperature gas flow cell developed at ISIS

R Haynes1 S T Norberg1 2 S G Eriksson2 M A H Chowdhury1 C M Goodway1

G D Howells1 O Kirichek1 and S Hull1 1 ISIS Facility STFC Rutherford Appleton Laboratory Chilton Didcot Oxfordshire OX11 0QX UK 2 Department of Chemical and Biological Engineering Chalmers University of Technology SE-412 96 Gothenburg Sweden E-mail olegkirichekstfcacuk Abstract A flow-through quartz gas cell together with a gas flow control and monitoring system has been designed and constructed at ISIS This equipment allows neutron powder diffraction data to be collected on samples at temperatures up to around 1300 K when exposed to user chosen mixtures of O2 Ar CO2 and CO By exploiting the sensitivity of neutrons to the presence of light atoms such as oxygen it is possible to probe the crystal structure of oxide materials as a function of oxygen partial pressures down to log10p(O2) of about -20 The resultant structural information can then be correlated with the bulk properties of the materials whose research and technological interests lie in fields such as energy production storage materials catalysis and earth science

1 Introduction Neutron powder diffraction is a well established tool for investigations of the structural properties of crystalline solids with the widespread availability of sample environment devices which allow studies to be performed as a function of temperature pressure and magnetic field (see for example [1]) Facilities which permit diffraction studies to be performed under controlled atmospheres have been developed at a number of neutron sources (see for example [2-3]) but tend to be rather less routinely used Nevertheless they offer an extremely powerful approach to probe the relationship between crystal structure and bulk properties of materials particularly if the latter are simultaneously monitored [4] This paper describes the design and construction of a flow-through quartz gas cell for use at ISIS together with its complementary flow control and monitoring system which allows the oxygen partial pressure within a flowing gas stream around the sample to be varied Its use is illustrated using data collected on nonstoichiometric ceria CeO2 using the Polaris medium resolution powder diffractometer [5]

2 Technical aspects Figure 1 illustrates the gas flow quartz cell and its location within the lsquoLeicesterrsquo furnace used at ISIS With reference to figure 1 the furnace uses a cylindrical vanadium foil heater element (4) surrounded by three vanadium heat shields (5) which minimise radiation losses to the outer jacket Modifications to this furnace were necessary to allow for the location of the gas flow quartz cell (6) These

International Conference on Neutron Scattering 2009 IOP PublishingJournal of Physics Conference Series 251 (2010) 012090 doi1010881742-65962511012090

ccopy 2010 IOP Publishing Ltd 1

modifications include the addition of a bottom flange which houses two lsquoOrsquo-rings to seal the quartz cell the thermocouple feed-through (9) to allow the thermocouple (type K used to control the temperature of the furnace) to be placed inside the quartz cell the gas inlet (2) and an increased vanadium heater element diameter to accommodate the quartz cell The top flange locates onto the NW40 flange on the top plate of the furnace and has three functions to hold and seal the quartz cell by means of a single lsquoOrsquo-ring to allow the gas to flow out (3) and to hold the oxygen and temperature sensor (type R) (1) The oxygen sensor (type DS zirconia sensor supplied by Australian Oxytrol Systems) is located just above the sample pellets (7) which sit on a porous quartz frit (pore size 16-40 m) The oxygen sensor monitors the content of O2 within the atmosphere flowing past the sample pellets with the oxygen partial pressure p(O2) (in units of atm) calculated using the voltage V (mV) and the temperature T (K) of the sensor using the Nernst equation p(O2) = pref(O2)e

46421VT where pref(O2) is the oxygen content within the reference air (pref(O2) = 0209)

Figure 1 The schematic diagram of the flow-through gas cell (with the right hand portion highlighting the central portion of the left hand figure) showing (1) oxygen and temperature sensor (2) gas inlet (3) gas outlet (4) heater element (5) heat shielding (6) quartz sample holder (7) sample (8) quartz frit (9) thermocouple

The composition and flow of gas through the quartz cell are controlled by means of a gas panel constructed at ISIS which consists of four mass flow controllers (Hastings 300 series) For the initial testing and commissioning of the apparatus the four gases used were O2 Ar CO2 and CO Each mass flow controller is controlled by a single channel digital display and can be set manually or via a Labviewreg user interface running on the powder diffractometerrsquos control computer The latter allows changes in the gas flow and composition to be synchronised with the neutron data collection so that powder diffraction spectra can be collected as the atmosphere surrounding the sample (and hence the oxygen partial pressure) are varied A pressure transducer and Baratronreg monitor the pressure in the

International Conference on Neutron Scattering 2009 IOP PublishingJournal of Physics Conference Series 251 (2010) 012090 doi1010881742-65962511012090

2

system and the panel and cell gas circuit are fitted with relief valves to protect the system from the over pressure

The assembly of the cell for a neutron experiment proceeds as follows Firstly the sample pellets are loaded in the quartz cell which is then placed inside the furnace Once located in the bottom flange and the lsquoOrsquo-ring nut tightened the top flange can be fitted The thermocouple is then pushed through the feed-through until it touches the underside of the quartz frit and the lsquoOrsquo-ring nut is tightened The oxygen sensor is inserted into the quartz cell and is sealed down by means of an lsquoOrsquo-ring and brass nut At this point the sample circuit is checked for leaks and the furnace placed into its outer jacket and evacuated Once loaded onto the neutron diffractometer the gas panel with the desired gases can be connected via a capillary line The capillary joint is checked for leaks and the system is ready to introduce gas The furnace can subsequently be heated to the required temperature at a rate dependent upon the furnace vacuum Cooling of the furnace is achieved by reducing the maximum power output and at approximately 400 K helium can be introduced to the furnace vacuum to speed up the cooling process

3 Preliminary Results Initial tests of the quartz cell and gas panel were performed on the Polaris powder diffractometer at ISIS [5] using a stack of 20 pellets of CeO2 each of 6 mm diameter and 2-3 mm thickness Diffraction data were collected at a temperature of 1273(2) K as a function of oxygen partial pressure with a gas flow of 80 cubic centimetres per minute and a succession of gas mixtures in the sequence O2ArCO2CO used to provide increasingly reducing atmospheres (see figure 2) Data collection times of 15 minutes were used with data collection started 5 minutes after changing the gas composition to allow equilibration

d-spacing (Aring)

05 10 15 20 25 30

Neu

tron

cou

nts

(arb

itra

ry u

nits

) decreasing p(O

2 ) larr

Figure 2 The neutron powder diffraction pattern of CeO2 measured at 1273(2) K on decreasing oxygen partial pressure p(O2) From bottom to top the spectra were obtained at log10p(O2) values of -01 -33 -83 -102 -121 -141 -148 -155 -164 -170 and -176

International Conference on Neutron Scattering 2009 IOP PublishingJournal of Physics Conference Series 251 (2010) 012090 doi1010881742-65962511012090

3

CeO2 and its chemical derivatives are of current interest as possible electrolyte materials for Solid

Oxide Fuel Cells (SOFCs) [6] Within an SOFC the electrolyte operates at temperatures in the range from 800 K to 1200 K and transmits O2 ions from the cathode (air side) to the anode (fuel side) Thus the material must be stable under both oxidizing and reducing atmospheres The data shown in figure 2 illustrate a potential drawback associated with the use of CeO2 because at oxygen partial pressures less than around 1014 atm there is a gradual increase in lattice parameter (associated with the reduction of some Ce4 to the larger Ce3 species) a loss of oxygen (which causes the changes in the relative intensities of the Bragg peaks such as the pair at ~125 Aring in figure 2) and the onset of significant disorder within the crystalline lattice (seen as a reduction in the intensities of the Bragg peaks at low d-spacings) The latter also cause diffuse scattering observed as weak broad undulating features in the intensity between the Bragg peaks (and not visible in figure 2) A detailed analysis of the diffuse scattering to probe the nature of the ionic disorder within reduced CeO2 has recently been published [7]

4 Conclusions Many common cation species such as Fe Cu Co and Mn can adopt different valence states As a consequence oxides containing these species often exhibit significant anion non-stoichiometry which can have a profound influence on the bulk properties of the material Thus in addition to the field of SOFC materials mentioned above the apparatus described in this paper is likely to find widespread applications within research areas such as high temperature superconductors multiferroic compounds and mineralogy A number of development projects are also underway to expand the range of science to be addressed including the use of controlled humid atmospheres for the study of proton conducting ceramics for fuel cell applications and the combined use of the gas panel with an in-situ resistance measurement cell [4] to simultaneously probe the structural and conducting properties of materials as a function of oxygen partial pressure and temperature

5 Acknowledgements The gas flow cell and control panel were constructed with financial support from the Swedish Research Council Vetenskapsraringdet

References [1] Bailey I F 2003 Z Kristallogr 218 84 [2] Turner J F C Done R Dreyer J David W I F and Catlow C R A 1999 Rev Sci Instrum 70 2325 [3] Tonus F Bahout M Henry P F Dutton S E Roisnel T and Battle P D 2009 Chem Comm 2556 [4] Engin T E Powell A V Haynes R Chowdhury M A H Goodway C M Done R Kirichek O and

Hull S 2008 Rev Sci Instrum 79 095104 [5] Hull S Smith R I David W I F Hannon A C Mayers J and Cywinski R 1992 Physica B 180-181

1000 [6] Ishihara T Sammes N M and Yamamoto O 2003 lsquoElectrolytesrsquo in lsquoHigh Temperature Solid Oxide

Fuel Cells Fundamentals Designs and Applicationsrsquo ed Singhal S C and Kendall K Elsevier Oxford

[7] Hull S Norberg S T Ahmed I Eriksson S G Marrocchelli D and Madden P A 2009 J Solid State Chem 182 2815

International Conference on Neutron Scattering 2009 IOP PublishingJournal of Physics Conference Series 251 (2010) 012090 doi1010881742-65962511012090

4

modifications include the addition of a bottom flange which houses two lsquoOrsquo-rings to seal the quartz cell the thermocouple feed-through (9) to allow the thermocouple (type K used to control the temperature of the furnace) to be placed inside the quartz cell the gas inlet (2) and an increased vanadium heater element diameter to accommodate the quartz cell The top flange locates onto the NW40 flange on the top plate of the furnace and has three functions to hold and seal the quartz cell by means of a single lsquoOrsquo-ring to allow the gas to flow out (3) and to hold the oxygen and temperature sensor (type R) (1) The oxygen sensor (type DS zirconia sensor supplied by Australian Oxytrol Systems) is located just above the sample pellets (7) which sit on a porous quartz frit (pore size 16-40 m) The oxygen sensor monitors the content of O2 within the atmosphere flowing past the sample pellets with the oxygen partial pressure p(O2) (in units of atm) calculated using the voltage V (mV) and the temperature T (K) of the sensor using the Nernst equation p(O2) = pref(O2)e

46421VT where pref(O2) is the oxygen content within the reference air (pref(O2) = 0209)

Figure 1 The schematic diagram of the flow-through gas cell (with the right hand portion highlighting the central portion of the left hand figure) showing (1) oxygen and temperature sensor (2) gas inlet (3) gas outlet (4) heater element (5) heat shielding (6) quartz sample holder (7) sample (8) quartz frit (9) thermocouple

The composition and flow of gas through the quartz cell are controlled by means of a gas panel constructed at ISIS which consists of four mass flow controllers (Hastings 300 series) For the initial testing and commissioning of the apparatus the four gases used were O2 Ar CO2 and CO Each mass flow controller is controlled by a single channel digital display and can be set manually or via a Labviewreg user interface running on the powder diffractometerrsquos control computer The latter allows changes in the gas flow and composition to be synchronised with the neutron data collection so that powder diffraction spectra can be collected as the atmosphere surrounding the sample (and hence the oxygen partial pressure) are varied A pressure transducer and Baratronreg monitor the pressure in the

International Conference on Neutron Scattering 2009 IOP PublishingJournal of Physics Conference Series 251 (2010) 012090 doi1010881742-65962511012090

2

system and the panel and cell gas circuit are fitted with relief valves to protect the system from the over pressure

The assembly of the cell for a neutron experiment proceeds as follows Firstly the sample pellets are loaded in the quartz cell which is then placed inside the furnace Once located in the bottom flange and the lsquoOrsquo-ring nut tightened the top flange can be fitted The thermocouple is then pushed through the feed-through until it touches the underside of the quartz frit and the lsquoOrsquo-ring nut is tightened The oxygen sensor is inserted into the quartz cell and is sealed down by means of an lsquoOrsquo-ring and brass nut At this point the sample circuit is checked for leaks and the furnace placed into its outer jacket and evacuated Once loaded onto the neutron diffractometer the gas panel with the desired gases can be connected via a capillary line The capillary joint is checked for leaks and the system is ready to introduce gas The furnace can subsequently be heated to the required temperature at a rate dependent upon the furnace vacuum Cooling of the furnace is achieved by reducing the maximum power output and at approximately 400 K helium can be introduced to the furnace vacuum to speed up the cooling process

3 Preliminary Results Initial tests of the quartz cell and gas panel were performed on the Polaris powder diffractometer at ISIS [5] using a stack of 20 pellets of CeO2 each of 6 mm diameter and 2-3 mm thickness Diffraction data were collected at a temperature of 1273(2) K as a function of oxygen partial pressure with a gas flow of 80 cubic centimetres per minute and a succession of gas mixtures in the sequence O2ArCO2CO used to provide increasingly reducing atmospheres (see figure 2) Data collection times of 15 minutes were used with data collection started 5 minutes after changing the gas composition to allow equilibration

d-spacing (Aring)

05 10 15 20 25 30

Neu

tron

cou

nts

(arb

itra

ry u

nits

) decreasing p(O

2 ) larr

Figure 2 The neutron powder diffraction pattern of CeO2 measured at 1273(2) K on decreasing oxygen partial pressure p(O2) From bottom to top the spectra were obtained at log10p(O2) values of -01 -33 -83 -102 -121 -141 -148 -155 -164 -170 and -176

International Conference on Neutron Scattering 2009 IOP PublishingJournal of Physics Conference Series 251 (2010) 012090 doi1010881742-65962511012090

3

CeO2 and its chemical derivatives are of current interest as possible electrolyte materials for Solid

Oxide Fuel Cells (SOFCs) [6] Within an SOFC the electrolyte operates at temperatures in the range from 800 K to 1200 K and transmits O2 ions from the cathode (air side) to the anode (fuel side) Thus the material must be stable under both oxidizing and reducing atmospheres The data shown in figure 2 illustrate a potential drawback associated with the use of CeO2 because at oxygen partial pressures less than around 1014 atm there is a gradual increase in lattice parameter (associated with the reduction of some Ce4 to the larger Ce3 species) a loss of oxygen (which causes the changes in the relative intensities of the Bragg peaks such as the pair at ~125 Aring in figure 2) and the onset of significant disorder within the crystalline lattice (seen as a reduction in the intensities of the Bragg peaks at low d-spacings) The latter also cause diffuse scattering observed as weak broad undulating features in the intensity between the Bragg peaks (and not visible in figure 2) A detailed analysis of the diffuse scattering to probe the nature of the ionic disorder within reduced CeO2 has recently been published [7]

4 Conclusions Many common cation species such as Fe Cu Co and Mn can adopt different valence states As a consequence oxides containing these species often exhibit significant anion non-stoichiometry which can have a profound influence on the bulk properties of the material Thus in addition to the field of SOFC materials mentioned above the apparatus described in this paper is likely to find widespread applications within research areas such as high temperature superconductors multiferroic compounds and mineralogy A number of development projects are also underway to expand the range of science to be addressed including the use of controlled humid atmospheres for the study of proton conducting ceramics for fuel cell applications and the combined use of the gas panel with an in-situ resistance measurement cell [4] to simultaneously probe the structural and conducting properties of materials as a function of oxygen partial pressure and temperature

5 Acknowledgements The gas flow cell and control panel were constructed with financial support from the Swedish Research Council Vetenskapsraringdet

References [1] Bailey I F 2003 Z Kristallogr 218 84 [2] Turner J F C Done R Dreyer J David W I F and Catlow C R A 1999 Rev Sci Instrum 70 2325 [3] Tonus F Bahout M Henry P F Dutton S E Roisnel T and Battle P D 2009 Chem Comm 2556 [4] Engin T E Powell A V Haynes R Chowdhury M A H Goodway C M Done R Kirichek O and

Hull S 2008 Rev Sci Instrum 79 095104 [5] Hull S Smith R I David W I F Hannon A C Mayers J and Cywinski R 1992 Physica B 180-181

1000 [6] Ishihara T Sammes N M and Yamamoto O 2003 lsquoElectrolytesrsquo in lsquoHigh Temperature Solid Oxide

Fuel Cells Fundamentals Designs and Applicationsrsquo ed Singhal S C and Kendall K Elsevier Oxford

[7] Hull S Norberg S T Ahmed I Eriksson S G Marrocchelli D and Madden P A 2009 J Solid State Chem 182 2815

International Conference on Neutron Scattering 2009 IOP PublishingJournal of Physics Conference Series 251 (2010) 012090 doi1010881742-65962511012090

4

system and the panel and cell gas circuit are fitted with relief valves to protect the system from the over pressure

The assembly of the cell for a neutron experiment proceeds as follows Firstly the sample pellets are loaded in the quartz cell which is then placed inside the furnace Once located in the bottom flange and the lsquoOrsquo-ring nut tightened the top flange can be fitted The thermocouple is then pushed through the feed-through until it touches the underside of the quartz frit and the lsquoOrsquo-ring nut is tightened The oxygen sensor is inserted into the quartz cell and is sealed down by means of an lsquoOrsquo-ring and brass nut At this point the sample circuit is checked for leaks and the furnace placed into its outer jacket and evacuated Once loaded onto the neutron diffractometer the gas panel with the desired gases can be connected via a capillary line The capillary joint is checked for leaks and the system is ready to introduce gas The furnace can subsequently be heated to the required temperature at a rate dependent upon the furnace vacuum Cooling of the furnace is achieved by reducing the maximum power output and at approximately 400 K helium can be introduced to the furnace vacuum to speed up the cooling process

3 Preliminary Results Initial tests of the quartz cell and gas panel were performed on the Polaris powder diffractometer at ISIS [5] using a stack of 20 pellets of CeO2 each of 6 mm diameter and 2-3 mm thickness Diffraction data were collected at a temperature of 1273(2) K as a function of oxygen partial pressure with a gas flow of 80 cubic centimetres per minute and a succession of gas mixtures in the sequence O2ArCO2CO used to provide increasingly reducing atmospheres (see figure 2) Data collection times of 15 minutes were used with data collection started 5 minutes after changing the gas composition to allow equilibration

d-spacing (Aring)

05 10 15 20 25 30

Neu

tron

cou

nts

(arb

itra

ry u

nits

) decreasing p(O

2 ) larr

Figure 2 The neutron powder diffraction pattern of CeO2 measured at 1273(2) K on decreasing oxygen partial pressure p(O2) From bottom to top the spectra were obtained at log10p(O2) values of -01 -33 -83 -102 -121 -141 -148 -155 -164 -170 and -176

International Conference on Neutron Scattering 2009 IOP PublishingJournal of Physics Conference Series 251 (2010) 012090 doi1010881742-65962511012090

3

CeO2 and its chemical derivatives are of current interest as possible electrolyte materials for Solid

Oxide Fuel Cells (SOFCs) [6] Within an SOFC the electrolyte operates at temperatures in the range from 800 K to 1200 K and transmits O2 ions from the cathode (air side) to the anode (fuel side) Thus the material must be stable under both oxidizing and reducing atmospheres The data shown in figure 2 illustrate a potential drawback associated with the use of CeO2 because at oxygen partial pressures less than around 1014 atm there is a gradual increase in lattice parameter (associated with the reduction of some Ce4 to the larger Ce3 species) a loss of oxygen (which causes the changes in the relative intensities of the Bragg peaks such as the pair at ~125 Aring in figure 2) and the onset of significant disorder within the crystalline lattice (seen as a reduction in the intensities of the Bragg peaks at low d-spacings) The latter also cause diffuse scattering observed as weak broad undulating features in the intensity between the Bragg peaks (and not visible in figure 2) A detailed analysis of the diffuse scattering to probe the nature of the ionic disorder within reduced CeO2 has recently been published [7]

4 Conclusions Many common cation species such as Fe Cu Co and Mn can adopt different valence states As a consequence oxides containing these species often exhibit significant anion non-stoichiometry which can have a profound influence on the bulk properties of the material Thus in addition to the field of SOFC materials mentioned above the apparatus described in this paper is likely to find widespread applications within research areas such as high temperature superconductors multiferroic compounds and mineralogy A number of development projects are also underway to expand the range of science to be addressed including the use of controlled humid atmospheres for the study of proton conducting ceramics for fuel cell applications and the combined use of the gas panel with an in-situ resistance measurement cell [4] to simultaneously probe the structural and conducting properties of materials as a function of oxygen partial pressure and temperature

5 Acknowledgements The gas flow cell and control panel were constructed with financial support from the Swedish Research Council Vetenskapsraringdet

References [1] Bailey I F 2003 Z Kristallogr 218 84 [2] Turner J F C Done R Dreyer J David W I F and Catlow C R A 1999 Rev Sci Instrum 70 2325 [3] Tonus F Bahout M Henry P F Dutton S E Roisnel T and Battle P D 2009 Chem Comm 2556 [4] Engin T E Powell A V Haynes R Chowdhury M A H Goodway C M Done R Kirichek O and

Hull S 2008 Rev Sci Instrum 79 095104 [5] Hull S Smith R I David W I F Hannon A C Mayers J and Cywinski R 1992 Physica B 180-181

1000 [6] Ishihara T Sammes N M and Yamamoto O 2003 lsquoElectrolytesrsquo in lsquoHigh Temperature Solid Oxide

Fuel Cells Fundamentals Designs and Applicationsrsquo ed Singhal S C and Kendall K Elsevier Oxford

[7] Hull S Norberg S T Ahmed I Eriksson S G Marrocchelli D and Madden P A 2009 J Solid State Chem 182 2815

International Conference on Neutron Scattering 2009 IOP PublishingJournal of Physics Conference Series 251 (2010) 012090 doi1010881742-65962511012090

4

CeO2 and its chemical derivatives are of current interest as possible electrolyte materials for Solid

Oxide Fuel Cells (SOFCs) [6] Within an SOFC the electrolyte operates at temperatures in the range from 800 K to 1200 K and transmits O2 ions from the cathode (air side) to the anode (fuel side) Thus the material must be stable under both oxidizing and reducing atmospheres The data shown in figure 2 illustrate a potential drawback associated with the use of CeO2 because at oxygen partial pressures less than around 1014 atm there is a gradual increase in lattice parameter (associated with the reduction of some Ce4 to the larger Ce3 species) a loss of oxygen (which causes the changes in the relative intensities of the Bragg peaks such as the pair at ~125 Aring in figure 2) and the onset of significant disorder within the crystalline lattice (seen as a reduction in the intensities of the Bragg peaks at low d-spacings) The latter also cause diffuse scattering observed as weak broad undulating features in the intensity between the Bragg peaks (and not visible in figure 2) A detailed analysis of the diffuse scattering to probe the nature of the ionic disorder within reduced CeO2 has recently been published [7]

4 Conclusions Many common cation species such as Fe Cu Co and Mn can adopt different valence states As a consequence oxides containing these species often exhibit significant anion non-stoichiometry which can have a profound influence on the bulk properties of the material Thus in addition to the field of SOFC materials mentioned above the apparatus described in this paper is likely to find widespread applications within research areas such as high temperature superconductors multiferroic compounds and mineralogy A number of development projects are also underway to expand the range of science to be addressed including the use of controlled humid atmospheres for the study of proton conducting ceramics for fuel cell applications and the combined use of the gas panel with an in-situ resistance measurement cell [4] to simultaneously probe the structural and conducting properties of materials as a function of oxygen partial pressure and temperature

5 Acknowledgements The gas flow cell and control panel were constructed with financial support from the Swedish Research Council Vetenskapsraringdet

References [1] Bailey I F 2003 Z Kristallogr 218 84 [2] Turner J F C Done R Dreyer J David W I F and Catlow C R A 1999 Rev Sci Instrum 70 2325 [3] Tonus F Bahout M Henry P F Dutton S E Roisnel T and Battle P D 2009 Chem Comm 2556 [4] Engin T E Powell A V Haynes R Chowdhury M A H Goodway C M Done R Kirichek O and

Hull S 2008 Rev Sci Instrum 79 095104 [5] Hull S Smith R I David W I F Hannon A C Mayers J and Cywinski R 1992 Physica B 180-181

1000 [6] Ishihara T Sammes N M and Yamamoto O 2003 lsquoElectrolytesrsquo in lsquoHigh Temperature Solid Oxide

Fuel Cells Fundamentals Designs and Applicationsrsquo ed Singhal S C and Kendall K Elsevier Oxford

[7] Hull S Norberg S T Ahmed I Eriksson S G Marrocchelli D and Madden P A 2009 J Solid State Chem 182 2815

International Conference on Neutron Scattering 2009 IOP PublishingJournal of Physics Conference Series 251 (2010) 012090 doi1010881742-65962511012090

4