Materials Research Bulletin 65 (2015) 1–6

Structural phase transitions of ionic layered PbFX (X = Cl�or Br–)compounds under high pressure

Y.A. Sorb *,1, D. Sornadurai, Condensed Matter Physics Division, Materials Science GroupIndira Gandhi Centre for Atomic Research, Kalpakkam 603102, India

A R T I C L E I N F O

Article history:Received 21 June 2014Received in revised form 30 September 2014Accepted 20 December 2014Available online 3 January 2015

Keywords:A. Layered compoundsC. High pressureC. X-ray diffractionD. Crystal structure

A B S T R A C T

The PbFX (X = Cl–or Br–) compounds crystallize in tetragonal structure with space group P4/nmm. Highpressure X-ray diffraction studies carried out on PbFCl compound reveals that it undergoes pressureinduced structural transitions at �18 GPa and �38 GPa to orthorhombic and monoclinic (P21/m) phasesrespectively. Like PbFCl, a similar phase transition from tetragonal to orthorhombic phase is observed inPbFBr at intermediate pressure. These phase transitions seem to be similar to the transitions involvingother matlockite structure compounds such as BaFX (X = Cl–, Br–or I–). PbFCl has a larger structuralstability range compared to BaFCl and is attributed to the large anisotropic coordination of the Pb2+ andCl–ions.

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1. Introduction

Ionic layered compounds of the form MFX, where M is adivalent metallic cation (Ca2+, Sr2+, Ba2+, Pb2+ or Eu2+) and X = Cl–,Br–or I–, crystallize in tetragonal structure with space groupP4/nmm [1]. A typical unit cell of MFX has two formula units inwhich the arrangement of atomic layers perpendicular to thec-axis is in the following sequence

F––M2+–X––X––M2+–F–

The unit cell of PbFCl compound (matlockite) is shown in Fig. 1.Apart from MFX compounds, two other families of compoundsnamely oxide halides [MOX] and hydride halides [MHX] crystallizein PbFCl-type structure. For example, in MOX compounds, thesequence of the layers is X–M–O–O–M–X [2], where the O layer isdoubly occupied with respect to the M and X layers, which issimilar to the sequence used by Yedukondalu et al. in BaFCl [3].However, the phase transitions of matlockite compounds underhigh pressure can easily be understood if we assume the sequenceof layers as F–M–X–X–M–F rather than X–M–F–F–M–X [3].

The matlockite compounds have interesting properties such asphotoconductivity [4], photoluminescence [5,6] and anisotropic

* Corresponding author. Tel.: +91 8022082963.E-mail address: sorbya@jncasr.ac.in (Y.A. Sorb).

1 Presently at Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur560 064, India.

http://dx.doi.org/10.1016/j.materresbull.2014.12.0550025-5408/ã 2015 Published by Elsevier Ltd.

ionic conductivity [7,8] and find application in various fields.For example, BaFBr doped with Eu2+ (BaFBr: Eu2+) has beensuccessfully applied for detecting X-rays as imaging phosphorsdue to the photo stimulated luminescence property [9].Similarly, BaFCl: Sm2+ has been used as a pressure sensor in highpressure experiments [10]. Based on a detailed luminescencestudy, pure PbFCl has been demonstrated to be an excellentscintillation detector material for neutrino detection [11].Effect of pressure on scintillator materials can be used toelucidate the correlation between the intrinsic resolution andtheir electronic band structure thereby one can design newerand better storage phosphors. But more theoretical and experi-mental evidences are required to validate the above statement.The high pressure structural behavior of matlockite structurecompounds such as BaFCl, BaFBr and BaFI have been extensivelystudied [12–20]. Using first principle calculation, the structural,electronic, and optical properties of the alkaline-earth halofluorides, BaXF (X = Cl, Br and I) and their high pressure structuralbehavior is extensively studied [3,21]. These systems undergo aseries of symmetry lowering structural transitions: tetragonal !orthorhombic ! monoclinic and have been attributed to a gradualanisotropic distortion of the charge distribution in the planesperpendicular to the stacking direction [13–15]. Replacement ofBa2+ with Pb2+ can be expected to result in stronger attractionbetween the two adjacent Cl–or Br– layers on account of the largerelectro negativity of Pb2+ ion (Fig. 1). It is interesting, therefore, tosee if PbFX (X = Cl–or Br–) exhibit a similar structural sequenceinduced by pressure and study their stability range. High pressure

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X-ray diffraction studies have been reported on the PbFBrcompound by Decremps et al. and have not observed any phasetransition up to �30 GPa [22]. Our recent high pressure work onPbFCl shows that two interlayer ‘rigid layer’ modes, namely A1 g

and Eg(1), of the parent tetragonal layered phase, suffer frominstability above �24 GPa as a result of pressure induced change ofbonding nature from layer to non-layer type [23]. This work hasfocused only on the dynamics of phonon modes of the PbFClcompound under pressure. Here, we present the detailed XRDstudies of the PbFCl compound to understand the pressureinduced structural behavior of PbFCl and also our results on PbFBrat �15 GPa.

2. Experimental details

The PbFX (X = Cl–or Br–) compounds were synthesized by thestoichiometric mixtures of PbF2 and PbX2.nH2O (n � 2) in nitrogenatmosphere using the solid state reaction method. Powder X-raydiffraction pattern of PbFCl showed that the compound was in asingle phase with tetragonal matlockite structure (P4/nmm). Thelattice parameters obtained from the powder pattern were:a = 4. 097 (3) Å and c = 7. 224 (2) Å and these values were matchedwell with the Joint Committee for Powder Diffraction Standard(JCPDS) values of a = 4. 1104 (2) Å and c = 7. 2325 (5) Å [24].Similarly, the lattice parameters obtained for PbFBr were:a = 4. 189 (4) Å and c = 7. 597 (1) Å and were matched well withthe JCPDS values of a = 4.191 Å and c = 7. 591 Å [25]. A Mao-Bell typediamond anvil cell (DAC) with culet size of �500 mm in diameterwas used for high pressure experiments. A stainless steel gasketwas preindented to a thickness of 50 mm; a hole of diameter200 mm was drilled in its center and finely powdered samplewas loaded into the gasket hole. A 4:1 mixture of methanol andethanol was used as pressure transmitting medium. Pressurewas determined using the equation of state of gold and Pt whichwere loaded along with PbFCl and PbFBr compounds respectively.HPXRD experiments were performed on the samples in angledispersive geometry using Mo Ka1 X-ray radiation obtainedfrom an 18 kW rotating anode X-ray generator and was mono-chromatized by a graphite monochromator. The sample to detectordistance was calibrated using LaB6. The diffraction patterns were

Fig. 1. Unit cell of PbFCl compound. The spheres with the labels F–, Pb2+ andCl–represent the type of ions.

collected using a mar345dtb diffractometer with an overallresolution of dd/d � 0.001 and the average acquisition time was2 h. The 2D XRD diffraction images were integrated usingFIT2D software which produces patterns of intensity versus thediffraction angle 2 u [26]. The HPXRD patterns of these compoundswere analyzed using Powd [27] and NBS-Aids*83 programs [28]to obtain the lattice parameters. Peak positions were determinedby a Gaussian fitting of the diffracted lines.

3. Results and discussions

The representative XRD patterns of PbFCl and PbFBrcompounds at various pressures are shown in Fig. 2a and brespectively. In PbFCl compound, the XRD pattern at �18 GPashows the emergence of a new peak at 2 u �19� and theintensity of this peak is seen to increase with pressure. Forinstance, the intensity of (10 2) peak decreases, while that of(2 0 0) peak increases drastically as shown in Fig. 2a. The analysisof the HPXRD pattern at �21 GPa shows that the new structureis orthorhombic with lattice parameters a = 7.751 (1) Å, b = 3.346(5) Å and c = 3.838 (3) Å. The starting tetragonal phase is seen tocoexist with the high pressure orthorhombic phase up to

Fig. 2. (a) The angle dispersive X-ray diffraction spectra of PbFCl at variouspressures. The slanted arrow indicates the emergence of new peaks, whereas theupward arrow indicates the intensity enhancement of the new peaks. Au is thepressure marker. ‘g’ is the gasket peak. The intensity enhancement of the peak(3 0 1)O with pressure is represented as a dotted line. (b) X-ray diffraction pattern ofPbFBr at various pressures. The stick patterns from JCPDS match with the ambientpattern of PbFBr. Pt (111), Pt (2 0 0) and Pt (2 2 0) are the peaks obtained from“Pt”which is used as pressure calibrant.

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�38 GPa. Similarly, the analysis of HPXRD pattern of PbFBr at�15 GPa shows that the new structure is orthorhombic withlattice parameters a = 7.927 (1) Å, b = 3.4928 (5) Å and c = 4.0202(3) Å. The lattice parameters corresponding to the tetragonalphase of PbFBr at �15 GPa is a = 3.9905 Å and c = 6.9895 Å. Themissing of the above transition in PbFBr by Decremps et al. maybe due to the use of energy dispersive X-ray diffractiontechnique in their experiments [16]. The inherent problemsassociated with this technique are the escape peaks andfluorescence peaks (like Ba, Ka and Kb) which may interferewith the sample peaks. However, the angle-dispersive XRDtechnique, employed by us, does not pose these problems.

The indexing of parent and high pressure phases of PbFCland the intermediate phase of PbFBr compounds are shown inFig. 3a and b respectively. In PbFBr, significant changes in theintensities of XRD pattern are observed at �13 GPa. For instance,the intensity of (0 0 1) peak is seen to reduce slowly and vanishescompletely above �13 GPa. The (0 0 2) peak is prominent atambient pressure and its intensity reduces slowly upon pressuri-zing further and finally merges with (10 1) peak. The (11 2) peak is

Fig. 3. (a) The X-ray diffraction pattern of PbFCl at various pressures. The suffixes ‘T’, ‘O’,phases respectively. (b) The indexing of XRD pattern at ambient and at �15 GPa. The indwith subscripts “T” and “O” indicate the mixture of tetragonal and orthorhombic phases�13 GPa and �15 GPa. The slanted arrow with the index (12 0)O indicates the new peak.The (12 0)O peak along with the (2 0 2)T and (2 10)T peaks are fitted to Gaussian fitting

completely merged with Pt (111) peak at intermittent pressuresand is gradually resolving above �13 GPa. Up to 10 GPa, both (2 0 2)and (211) peaks are merged as a single peak and are well resolvedat �13 GPa. At this pressure, appearance of an additional peak at�24� is observed and is indexed as (12 0)O. The evolution of thispeak with pressure is shown in Fig. 3c where the dotted lineindicates the intensity enhancement of the peak with pressure.Above �15 GPa, the (12 0)O peak is completely merged with the(2 0 2) and (211) peaks. The HPXRD pattern of PbFBr at �15 GPashows that the compound undergoes a symmetry loweringstructural transition from the parent tetragonal structure to thelow symmetry orthorhombic structure.

When pressure is applied on PbFCl compound, there can be alarge compression of the electron charge density of Cl� ionsalong c-axis as compared to the a and b planes (Fig. 4). Thiscauses an anisotropic redistribution of the charge cloud alongthe a and b planes which leads to a gradual distortion(stretching of a-axis) of the parent tetragonal lattice [13–15].On further increasing pressure, around 38 GPa, a second newpeak emerges at 2 u = 12.2� (Fig. 2a) and the intensity

and ‘M’ to the (hkl) indices represent the tetragonal, orthorhombic and monoclinicices, with subscripts “T” indicate the starting tetragonal phase, whereas the indices. Pt (111), Pt (2 0 0) and Pt (2 2 0) represent Pt. (c) The HPXRD patterns of PbFBr at

The dotted line indicates the intensity enhancement of (1 2 0)O peak with pressure. to resolve the peaks.

Fig. 4. The black arrows indicate the possible dynamics of Pb2+, F� and Cl� ionsunder pressure. Thick red arrows indicate the movement of adjacent halide ions(X�) and the cation (Pb2+) under high pressure. The thick blue arrows represent theexpansion of the lattice along the a and b planes at high pressure. The light green,yellow and light blue represent X�, F� and Pb2+ ions respectively. The coordinateaxes represent the direction of lattice vectors. (For interpretation of the referencesto color in this figure legend, the reader is referred to the web version of this article.)

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enhancement of this peak with pressure as shown in Fig. 2a andFig. 3a. The (10 2) peak belonging to the starting tetragonalphase is seen to reduce in intensity drastically and almostvanishes at �47 GPa (Fig. 2a). The splitting of (10 1)T peak to adoublet at around 38 GPa indicates a signature of symmetrylowering structural transition (Fig. 3a). Furthermore, theintensity of this peak is seen to increase with pressure and isrepresented as a dotted line. At around 47 GPa, the tetragonalsymmetry is almost broken and the lattice is transformed tomonoclinic phase. The intensity of (3 0 1)O peak also increaseswith pressure (Fig. 2a and Fig. 3a).

The XRD pattern at �38 GPa was taken up for detailed analysis.The pattern could be indexed to a mixture of the tetragonal,orthorhombic and monoclinic phases. The monoclinic phase wasseen to be similar to that in BaFCl, namely having space groupP21/m. The lattice parameters for the three co-existing phasesat �38 GPa: aTetra = 3.720(3) Å and cTetra = 6.509(1) Å; aOrtho = 7.599(1) Å, bOrtho = 3.281(3) Å and cOrtho = 3.562(1) Å and aMono =7.340

Fig. 5. (a) Pressure dependence of lattice parameters and c/a ratio of tetragonal phase

10 GPa. (b) Pressure vs pressure derivative of lattice parameters of tetragonal phase of

(3) Å, bMono = 3.546(4) Å, cMono = 3.428(5) Å and bMono =107.86; withtwo formula units in the unit cell. Since the quality of data at highpressures is very poor, so we couldn’t perform Retvield refinementfor this system.

Thus the co-existence of phases over a wide pressure range istypical of systems that undergo gradual symmetry loweringtransition sequence, for example, like in BaFX systems [15,17,29].The analysis of HPXRD pattern of PbFBr at �15 GPa shows that theobserved phase transition is continuous rather than sharp similarto other matlockite systems [17,29].

It is interesting to study the mechanism of structural phasetransition in PbFCl under pressure. In PbFCl crystal lattice, theadjacent Cl� ion layers are bonded by weak ionic bonding. Thereexists an attraction of the Pb2+ ions with the counter Cl� and F�

anions. Similarly, there exists repulsion between the adjacentlayers containing chloride ions and also between fluoride ions(Figs. 1 and 4). The pressure dependence of a, c and c/a ratio oftetragonal lattice is shown in Fig. 5a. The pressure derivatives oflattice parameters (da/dp & dc/dp) of the tetragonal phase of PbFClare calculated by fitting the P vs a, c data to the second orderpolynomial. It is evident from Fig. 5a and b that for pressures below10 GPa, the variation of lattice parameter ‘c’ with pressure isslightly faster as compared to ‘a’. At higher pressures (beyond15 GPa), the lattice parameter ‘c’ increases much faster than that of‘a’ (Fig. 5b). This anisotropic compression of lattice parameters ofthe tetragonal lattice of PbFCl arises from the weak bonding ofadjacent chloride ion layers, which causes the redistribution of thecharge cloud of the Cl� ions in the a and b planes which in turncauses the gradual distortion of tetragonal lattice into a lowersymmetry orthorhombic lattice. The phase transition thusobserved is sluggish in nature due to the small Gibb’s free energydifference between the parent tetragonal and the intermediateorthorhombic phases [17]. On further pressurizing the lattice, theorthorhombic lattice gets tilted in the a and b planes which leads tothe phase transformation from orthorhombic to monoclinic phaseat �38 GPa [30]. Another possibility is that the tilting of thetetragonal phase in the a and b planes with respect to the c-axiscauses the phase transition from orthorhombic to monoclinic at�38 GPa. The phase transitions thus observed in PbFCl compoundis exactly in agreement with our recent work on high pressureRaman study [23,31]. The phase transitions thus observed isalso continuous rather than sharp due to the small Gibb’s freeenergy difference between these phases [17]. Fig. 6 shows the

of PbFCl. The inset represents the pressure dependence of lattice parameters up toPbFCl.

Fig. 6. P vs d (hkl) spacings of PbFCl before and after the phase transitions. The dottedlines at �18 GPa and �38 GPa indicate the emergence of new phases. The capitalbold letters T, O and M at the top of the graph represent the tetragonal,orthorhombic and monoclinic phases respectively. The indices, with subscripts ‘T’,‘O’ indicate the mixture of tetragonal and the orthorhombic phases. Similarly theindices with subscripts ‘T’, ‘O’ and ‘M’ indicate the mixture of tetragonal,orthorhombic and monoclinic phases respectively. The indices, with the subscript‘M’ indicate the monoclinic phase.

Table 1Experimental and computed bulk moduli of different matlockite compounds.

Compounds BaFCl Ref.[16]

BaFBr Ref.[15]

BaFI Ref.[15]

PbFCl(presentwork)

PbFBr Ref.[20]

Bulk moduli B0 = 45 � 3 B0 = 38 � 11 B0 = 47 � 6 B0 = 51 � 3 B0 = 39.9(4)

Experiment B0’ = 5.2 � 0.5 B0’ = 7.6 � 2 B00 = 5 � 0.5 B0’ = 5.6 B0’ = 7.7(1.5)

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variation of lattice spacings d(hkl) of PbFCl as a function of pressurebefore and after the transition. The lattice spacings varymonotonically with pressure.

The P–V curves for the tetragonal phase of PbFCl and PbFBrcompounds are shown in Fig. 7a and b respectively. The data arefitted to the third-order Birch–Murnaghan equation of state [32].The solid line is the best fit to the third-order Birch–Murnaghanequation of state. The bulk modulus and its first derivativeobtained for the tetragonal phase of PbFCl are BT = 51 �3 GPa andBT

’ = 5.6. Similarly, the experimentally obtained bulk moduli valuesfor the PbFBr compound is B0 = 39.9 � 4 GPa and B0’ = 7.7 � 1.5 andis comparable with the values B0 = 43 (15) and BT’ = 6 (3) obtainedby Decremps et al. [22]. These values are comparable to thecalculated bulk modulus of BT = 47 GPa and BT’ = 4.1 GPa using a

Fig. 7. (a) P–V data of the tetragonal phase of PbFCl. The solid line is the best fit to the thphase of PbFBr. The solid line is the best fit to the third-order Birch–Murnaghan equat

shell model [33]. The bulk modulus of PbFCl is compared withsimilar matlockite compounds as shown in Table 1. The bulkmodulus of PbFCl is marginally more than that of BaFCl and PbFBrcompounds which can be attributed to the large electro negativityof Pb (2.33 in Pauling units) in comparison to Ba (0.89 in Paulingunits) and Cl (3.16 in Pauling units) in comparison to Br (2.96 inPauling units), there by strong interlayer attraction leading to thelower compressibility of PbFCl [34]. For instance, the transitionpressures for the tetragonal ! orthorhombic ! monoclinic are10.8 and 21 GPa respectively for BaFCl whereas it is 18 and38 GPa respectively for PbFCl (Table 2). The structural stability ofPbFCl compound can be understood in terms of anisotropiccoordination and polarizability of Pb2+ and Cl– ions. Fig. 8 showsthe polyhedral representation of PbFCl compound. In this figure,the two apical Cl� ions in the octahedral are at different distancesfrom the Pb2+ ion. The nearest one is at a distance of 2.3525 Åwhereas the farthest one is at 3.9765 Å. However, the four Cl� ionsare at equidistance of 3.0993 Å from the Pb2+ ion. The anisotropicbonding forces between ions in this layered structure are related tothe non equidistant position of the halogen anion around thecations (here Pb2+). Therefore, the pressure, which affectsprimarily the weak bonds, which may lead to a redistribution ofthe halogen coordination. Pressure changes the size (or distance) ofPb2+ and X2� ions which is associated with distortions in thestructural framework leading to a more efficient packing of thelattice. Also during this process, a decrease in Madelung numberreported in BaFCl compound at low pressure [35]. Furthermore, therelative loss of ionic lattice energy is compensated by an increase ininteractions due to polarization. Since high pressure behaviors ofPbFCl and BaFCl compounds are almost similar except thetransition pressure range; hence the same arguments can beapplicable to PbFCl compound too.

ird-order Birch–Murnaghan equation of state. peaks. (b) P–V data of the tetragonalion of state.

Table 2Transition pressures of BaFCl and PbFCl compounds.

Transition pressures (GPa)

Compounds Ortho. Mono. Ref.

BaFCl 10.8 21 [14]PbFCl 18 38 Present workPbFBr 13 – Present work

Fig. 8. Polyhedral representation of PbFCl compound at ambient pressure. Theviolet grey, dark red and grey colors represent F�, Pb2+ and Cl� ions respectively. (Forinterpretation of the references to color in this figure legend, the reader is referredto the web version of this article.)

6 Y.A. Sorb, D. Sornadurai / Materials Research Bulletin 65 (2015) 1–6

4. Conclusion

PbFX (X = Cl–or Br–) exhibits interesting structural phasetransitions at high pressures. Like BaFCl, BaFBr and BaFIcompounds, PbFCl also undergoes a pressure induced structuralphase transition from the tetragonal phase to monoclinic phase(P21/m) via an intermediate orthorhombic phase. All the threephases are coexisting up to 38 GPa and around 47 GPa theorthorhombic lattice almost transforms to monoclinic lattice.The structural stability of PbFCl compound can be understood interms of anisotropic coordination and polarizability of Pb2+ and Cl–

ions. Similar to PbFCl, a symmetry lowering phase transition fromtetragonal to orthorhombic is seen in PbFBr at intermediate

pressure. The bulk moduli of these compounds are calculated andare compared with similar matlockite compounds.

Acknowledgments

The authors are grateful to P.Ch. Sahu and N. Subramanian forvaluable discussions. We acknowledge L. Meenakshi Sundaram fortechnical help. We are grateful to P.R. Vasudeva Rao, C.S. Sundar,M.P. Janawadkar and A.K. Arora for support and encouragement.N.V. Chandrashekar, N.R. Sanjay Kumar, T.R. Ravindran, R. JohnKennedy and M. Sekar are thanked for their help in various stagesof the work. We gratefully acknowledge financial support fromDAE, India.

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