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Intermetallics 19 (2011) 1622e1626

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Intermetallics

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Structure phase transitions of polymorphic compounds with layered crystalstructures: The REIr2Si2 case

M. Mihalik a,*, J. Pospí�sil b, A. Rudajevová b, X. Marti b, D. Wallacher a, A. Hoser a, T. Hofmann a, M. Divi�s b,V. Sechovský b

aHelmholtz-Centre Berlin for Materials and Energy, Hahn-Meitner Platz 1, D-14109 Berlin, GermanybDepartment of Condensed Matter Physics, Faculty of Mathematics and Physics, Charles University in Prague, Ke Karlovu 5, 121 16 Prague 2, Czech Republic

a r t i c l e i n f o

Article history:Received 27 May 2011Received in revised form6 June 2011Accepted 24 June 2011Available online 19 July 2011

Keywords:A. Rare-earth intermetallicsB. Phase transformationC. Heat treatmentF. Diffraction

* Corresponding author. Tel.: þ49 30 8062 42793;E-mail address: matus.mihalik@helmholtz-berlin.d

0966-9795/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.intermet.2011.06.011

a b s t r a c t

REIr2Si2 (RE ¼ La, Ce, Pr and Nd) were found to be polymorphic compounds which can crystallize in twodifferent tetragonal types of crystallographic structures (b-phase: CaBe2Ge2-type; a-phase: ThCr2Si2-type). Here we study the structural phase transitions (a- to b- and b- to a-) using thermal expansion,differential thermal analysis and neutron diffraction. Both phase transitions occur at temperatures lowerthan 1300 �C, have a hysteresis up to 300 �C, do not show any intermediate steps and are accompaniedwith the absorption/release of the latent heat. We conclude that these structural phase transitions are ofthe first order.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Understanding the role of crystal structure in determining thephysical properties of solids is a crucial issue of the design of newmaterials. Polymorphism, the ability of a material to adopt morethan one crystal structure while preserving the chemicalcomposition sometimes provides unique opportunities to studyrelationships between crystal symmetry and various physicalproperties. It is particularly convenient in case when the high-temperature (HT) crystallographic phase can be quenched downto low temperatures as a metastable phase whereas the stablelow-temperature (LT) phase is achieved as a result of sufficientlyslow cooling.

There have been reported several polymorphic compounds, inwhich LT and HT phase, respectively, can be stabilized at roomtemperature depending on the thermal treatment. In this manu-script we address the REIr2Si2 (RE¼ La, Ce, Pr, Nd and Yb) [1e5], butthe RENi2As2 compounds (RE ¼ La, Ce, Pr, Nd, Sm) [6] may serve asadditional examples. These compounds can crystallize either in thehigh-temperature b-phase (tetragonal CaBe2Ge2-type, space group

fax: þ49 30 8062 42999.e (M. Mihalik).

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P4/nmm) or in the low-temperature a-phase (ThCr2Si2-type, spacegroup I4/mmm) which differ by the stacking of the atoms along thec-axis (see Fig. 1). It should be emphasized that although the unitcell volumes of the two phases are almost identical, the corre-sponding values of the c/a ratio differ by as much as 5%, as illus-trated in Table 1.

The theoretical density functional calculations of total energyfor both phases in the case of LaIr2Si2, confirmed the a-phase asthe ground state phase [4]. Nevertheless, the b-phase can beachieved at low temperatures (room temperature and below) byrapid cooling (quenching) from the melt, growing a single crystalby Czochralski method [7], or by the flux growth from theproperly chosen flux [5]. The b-phase is, however, metastable atroom temperature and a relatively short time annealing atslightly elevated temperature transforms it completely to the a-phase (see for example case of PrIr2Si2 [4]). That may be thereason why some samples prepared by slow cooling [8] from hightemperatures to room temperature, appeared only in theThCr2Si2-type structure.

In our previous work [7,9e11] we have concentrated mostly onthe low-temperature magnetic properties of the two crystallo-graphic phases of Ce, Pr and Nd containing REIr2Si2 compounds. Inthe present paper we focus our attention to the structural phasetransformations between the a- and b-phase. For this purpose wehave studied the processes by in situ experiments, namely

Fig. 1. The sketch of the positions of the atoms in REIr2Si2 compounds in a) ThCr2Si2crystallographic structure and b) CaBe2Ge2 crystallographic structure.

M. Mihalik et al. / Intermetallics 19 (2011) 1622e1626 1623

differential thermal analysis (DTA), dilatometer studies and hightemperature neutron diffraction experiment.

2. Sample preparation and experimental details

Polycrystalline samples of REIr2Si2, (RE ¼ La, Ce, Pr, Nd) havebeen prepared by arc melting using a stoichiometric amounts ofstarting materials (purity at least 3 N) in a water-cooled coppercrucible under an Ar (6 N) protective atmosphere. Samples were re-melted several times to ensure good homogeneity. The 18 mm longcylindrical (6 mm in diameter) bars needed for the dilatometricexperiment have been prepared by casting the melt to a specialcrucible. All these samples will be referred in the following as as-cast samples. Based upon our previous work [4] it is expectedthat the majority fraction of the as-cast samples is the b-phase.While for the thermal expansion experiment we used the as-castsamples, for the neutron and DTA experiments, the samples havebeen annealed for 12 h at 500 �C in vacuum. The routine analysis ofthe latter samples by X-ray powder diffraction prior to the neutrondiffraction experiment revealed that they consist of only the a-phase.

The linear thermal expansion (TE) was measured with thesample in helium (He) atmosphere using the Netzsch 402E dila-tometer over the range from room temperature to 1300 �C. For each

Table 1The comparison of the lattice parameters of the REIr2Si2 compounds. The data takenfrom the literature were obtained by the x-ray experiments, while the data pre-sented in this work were obtained by the neutron experiment. R.T. means roomtemperature.

T (�C) a (nm) c(nm) c/a V (nm3) Ref.

a-CeIr2Si2 R.T. 0.40848(4) 10.158(6) 2.487 0.1695(1) [2]100 0.4115(2) 1.022(3) 2.484 0.1731 This work

b-CeIr2Si2 R.T. 0.41431(4) 0.986(5) 2.376 0.16910(9) [2]650 0.4192(2) 1.015(3) 2.421 0.1784 This work

a-PrIr2Si2 R. T. 0.40842(3) 1.0145(1) 2.484 0.1692 [4]250 0.4120(2) 1.024(3) 2.485 0.1738 This work

b-PrIr2Si2 R. T. 0.41522(3) 0.98471(8) 2.372 0.1698 [4]880 0.4210(2) 1.001(3) 2.378 0.1774 This work

a-NdIr2Si2 R.T. 0.4080(2) 1.0082(5) 2.471 0.1679 [11]150 0.4094(2) 1.012(3) 2.472 0.1696 This work

b-NdIr2Si2 R.T. 0.4144(4) 0.9832(2) 2.373 0.1688 [11]900 0.4200(2) 1.006(3) 2.395 0.1774 This work

sample we have performed two runs with the constant heating andcooling rate of �5 K/min, respectively. The onset of the anomaly ontemperature dependence of the coefficient of thermal expansion(CTE) curves will be considered as the temperature of the phasetransition. Before considering the CTE data it is worth mentioningthat during sample preparation, the sample is, on one side, cooledby the copper crucible and, on the opposite side, it is heated byelectrical arc. In this geometry, when the arc is switched off, thecooling of the melt starts from the copper crucible and continuestowards the hottest place of the sample. It is found that thisprocedure induces a preferential orientation into polycrystallinesamples. Both the texture and the large change of the c/a ratiobetween the a and b phase cause observation of TE anomaliesconnected with structural phase transitions despite the fact thatthe volume of the unit cell is the same for both the crystallographicphases.

DTA was measured on Setaram SETSYS-2400 CS instrumentwith the sample in He atmosphere applying the constant heating/cooling rate � 5 K/min, respectively. We have used polycrystallinesamples having a typical mass of 150 mg enclosed in the aluminacontainer. We chose the phase transition temperature as the onsetof the anomaly in the DTA signal.

The neutron diffraction experiment was performed using thewavelength of 2.438 Å on the E6 diffractometer (Helmholtz -CentreBerlin) with the HTF-I furnace. To prevent the unwanted oxidationof the sample we have used the solid polycrystalline rods (insteadof the powder sample) prepared in a similar way as for the dila-tometric experiment inserted into a Tantalum container. Underthese conditions, the virtually unpredictable preferential orienta-tion in the samples does not allow refining the obtained data bystandard Rietveld analysis. Since the possible crystal structures areknown, the position of the atoms in the particular structures(except of the zIr and zSi positions in the b-phase) is fixed by thesymmetry and the compounds are paramagnetic at the tempera-tures of the experiment, we have focused only on the temperatureevolution of the lattice parameters and the temperatures of thestructural phase transitions. We have extracted this informationfrom the 2Theta location of the peaks using LeBail procedure [12],also implemented in FullProf software package [13]. We havecollected the 20� < 2q < 132� scans at the low temperature (typi-cally around 100 �C) and at the highest temperature (between 1200and 1400 �C). During the heating-cooling excursions, we have usedthe two position-sensitive detectors of E6 to cover ranges45� < 2q < 60�; 70� < 2q < 85� and we have collected the data for15min at each temperature set points. Each set point was stabilizedfor 10 min prior to the measurement.

3. Results and discussion

At the beginning of the first TE measurement cycle, we haveobserved the elongation of the samples at 340 �C for LaIr2Si2, 430 �Cfor CeIr2Si2, 340 �C for PrIr2Si2 and 320 �C for NdIr2Si2 (see Fig. 2).We attribute this feature to the phase transformation of thequenched, metastable HT b-phase into a thermodynamically morestable state (a-phase). Note, that in case of PrIr2Si2, this effect is ingood agreement with the reported b- to a-phase relaxation effect[4]. The rest of the run exhibits the same features as the second runand thence we discuss directly the second run. In a second thermalcycle, two sharp anomalies on TE data (Fig. 3a and SupplementaryFig.1) are observed corresponding to the contraction (expansion) ofthe sample on the heating (cooling) curves. One can notice thesizeable hysteresis, up to 300 �C, for all the studied materials. Note,that after the second temperature run we have observed an irre-versible elongation of our samples (Fig. 3a). We estimate that thiseffect relates to the formation of microcracks. These microcracks

Fig. 2. Temperature dependence of the relative elongation for the first run for a) LaIr2Si2, b) CeIr2Si2, c) PrIr2Si2, d) NdIr2Si2.

M. Mihalik et al. / Intermetallics 19 (2011) 1622e16261624

evolve as a release of tension in the sample and during the third runthey grow to dimensions that they cause the rupture of thesamples.

DTA data (Fig. 3b and Supplementary Fig. 1) for all compoundsshow an endothermic peak (absorption of latent heat) on theheating and an exothermic peak (release of the latent heat) on thecooling scan. Both peaks match well to the anomalies observed onthe TE data and hence we have assigned these peaks to the studiedstructural phase transitions. Since latent heat is involved in thesestructural phase transitions, we estimate that both transitions areof the first order.

The neutron experiment has shown that, starting from the a-phase, this low temperature phase is stable up to 950e1150 �C,depending on the rare earth, showing only small, temperaturedriven increase of the lattice parameters (see SupplementaryFig. 2). The complete transformation takes place within less than10 �C (close to our temperature resolution) and showing, after-wards, only the expected small thermal expansion changes in thelattice parameters up to the highest accessible temperature. Thewhole set of 2q scans at highest temperature are consistent withthe extinction rules for P4/nmm space group. Therefore, we claimthat the b-phase forms at temperatures higher than 950e1150 �C,depending on the compound. The reverse transformation has beenfound during the cooling down to room temperature but revealinga wide hysteresis of 300 �C, in agreement with TE and DTAmeasurements. Interestingly enough, we note that the c/a ratio isalmost temperature-independent within the a- and b-phase;respectively (see Fig. 4). We have observed no intermediate phasesat the edges of these phase transitions.

To conclude the presentation of the experimental results, wesummarize in Fig. 5 the comparison of the transition temperaturesas determined by the different methods. We note that the corre-sponding transition temperatures determined by all three methods

are within the experimental error in good agreement despite thefact that two of them (DTA and TE) were measured witha temperature sweep and only one (neutron diffraction) wasmeasured at stable temperatures. In particular, in the threedifferent experimental techniques, we have observed the largehysteresis up to 300 �C in all studied compounds. This hysteresis,which indicates a first order phase transition, raises three impor-tant issues that we will address subsequently: the origin of thehysteresis, the sharpness of the phase transformations and theintriguing scaling of the transition temperatures with the atomicnumber of the rare earth.

The two polymorphic crystal structures in investigation arethe ordered ternary variants of the BaAl4-type tetragonal struc-ture. They are built up of RE, T and X basal-plane atomic layerspilled up along the c-axis. The main difference between the twotypes is the stacking along c, which is RE-X-T-X - RE - X-T-X-RE.for the ThCr2Si2-type and RE-T-X-T - RE - X-T-X-RE. for theCaBe2Ge2-type. Thus in the former structure type the RE andtransition metal atoms are separated by the X-atom layers,whereas the T- and X-atom layers are interchanged in one half ofthe unit cell in the latter one. Intuitively, one may expect that thecrystal phase transition between the two structures should costa lot of energy and consequently a large temperature hysteresiscan be envisaged.

R. Hoffmann and C. Zheng [14] performed calculations of theXeX bonds in the AB2X2 compounds with ThCr2Si2 crystallo-graphic structure. They concluded that for both possibilities, XeXbonds present (bond state) and XeX bonds broken (no-bondingstate), there exist the energy minimum. Thence for some A, B andX element configuration there may be a structural phase transi-tion. Our experiments presented here for the REIr2Si2 compoundsexperimentally confirm this hypothesis. The theoretical existenceof a double-well-like ground state explains why we were able to

Fig. 4. The temperature dependence of c/a ratio for a) CeIr2Si2, b) PrIr2Si2 and c)NdIr2Si2 as determined from the neutron experiment. The arrows indicate the direc-tion, by which the temperature was changed during the experiment.

Fig. 5. Excellent agreement among 3 different experimental techniques in thecomparison of the temperature of the structural phase transition for the studied RE ion.Filled points represent the a- to b-phase transition and opened points represent the b-to a-phase. The lines are guides to the eye.

b

a

Fig. 3. The representative results from the bulk experiments for the PrIr2Si2. a) therelative elongation for the second run, b) the DTA data. The arrows represent thetemperature sweep direction. The dashed lines represent the most probable positionsof the structural phase transitions. The similar results for the remaining threecompounds are provided as Supplementary Fig. 1.

M. Mihalik et al. / Intermetallics 19 (2011) 1622e1626 1625

observe exactly two and only two different crystallographicstructures separated by sharp phase transitions. Furthermore,based on the mentioned model [14], we can also proposea qualitative explanation of the large hysteresis in thesecompounds: at low temperatures the energy minimum for theSieSi short distance along the c-axis (a bond state in the a-phase,typically 2.3 Å long) occurs at lower energy than the minimumfor the corresponding long distance (a no-bond state in the b-phase, over 3.9 Å long). Therefore, while the temperature rises,the energy minimum for the no-bond state is lowered. However,due to the energy barrier between the two states, the compoundpersists in the a-phase structure (bond state). By furthertemperature increase, the bond state energy minimum getsshallower and the no-bond state energy minimum gets deeper,consequently the barrier between the two minima vanishes andthe compound transforms into the b-phase structure. Taking intoaccount identical construction for the cooling back to roomtemperature, we end up with the hysteretic behaviour. Finally,we may place a question whether the transition temperatures ofLaIr2Si2 are anomalously high, or the transition temperature ofCeIr2Si2 is anomalously low. Further investigations of morecompounds are required to address this issue.

M. Mihalik et al. / Intermetallics 19 (2011) 1622e16261626

4. Conclusions

We have studied the b- to a- and the a- to b- structural phasetransition in polymorphic REIr2Si2 (RE ¼ La, Ce, Pr and Nd)compounds. We have confirmed that the b-phase can be quenchedto room temperature, despite it is metastable and can be trans-formed to the a-phase by short annealing at slightly elevatedtemperatures.Wehave shownthat in all compounds there exist onlytwo different structural phases at temperatures lower than 1400 �C,in agreementwith theoretical predictions in similar alloys. In all fourstructural phase diagramswehave found a region of roughly 300 �C,where the compounds can adopt one of the two crystallographicphases, depending on, whether the region is approached fromhigher or lower temperatures. The DTA analysis has confirmed thatthere is latent heat connected with both phase transitions. Weconclude that the studied phase transitions are of the first order.

Acknowledgements

This work is a part of the research plan MSM0021620834financed by the Ministry of Education of the Czech Republic and bythe Czech Grant Agency (Project # 202/09/1027).

Appendix. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.intermet.2011.06.011.

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