Structures and Phase Transitions in Pertechnetates - ACS Publications

Jul 9, 2019 - the fission of uranium in nuclear reactors, having a 6.1% mass yield. In its 7+ valence ... Technetium is the lightest element in the pe...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Structures and Phase Transitions in Pertechnetates Brendan J. Kennedy,*,† Sean Injac,† Gordon J. Thorogood,‡ Helen E. A. Brand,§ and Frederic Poineau∥ †

School of Chemistry, F11 The University of Sydney, Sydney, New South Wales 2006, Australia Australian Nuclear Science and Technology Organisation, Lucas Heights, New South Wales 2234, Australia § Australian Synchrotron, Australian Nuclear Science and Technology Organisation, ANSTO, 800 Blackburn Road, Clayton, Victoria 3168, Australia ∥ University of Nevada Las Vegas, Department of Chemistry and Biochemistry, 4505 Maryland Parkway, Las Vegas, Nevada 89154. United States Downloaded via GUILFORD COLG on July 18, 2019 at 16:12:03 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: The temperature dependence of the structures of four pertechnetates (ATcO4 A = Ag, Tl, Rb, Cs) from 90 K to their melting points is described. Synchrotron X-ray diffraction measurements show that RbTcO4 undergoes a I41/a to I41/amd transition near 530 K that is associated with a change in the orientation of the TcO4− tetrahedra about the scheelite b axis. AgTcO4 also exhibits a tetragonal scheelite type structure, and this is retained between 90 and 750 K, above which it melted. CsTcO4 has an orthorhombic pseudo-scheelite structure at room temperature and this undergoes a first-order orthorhombic to tetragonal transformation (Pnma to I41/a) near 430 K. TlTcO4 is isostructural with CsTcO4 at 90 K, but the orthorhombic to tetragonal transformation proceeds via an intermediate orthorhombic phase. The different behavior found here and described previously for the analogous Re oxide TlReO4 highlights the differences in the chemistry of these two systems.

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INTRODUCTION Technetium is the chameleon of the nuclear industry. On one hand, 99mTc with a half-life of about 6 h is the most widely used radiopharmaceutical and its production has been identified as a key driver for the construction, and ongoing operation, of research reactors.1 On the other hand, 99Tc with a half-life of about 211000 years is an appreciable byproduct of the fission of uranium in nuclear reactors, having a 6.1% mass yield. In its 7+ valence state Tc forms the pertechnetate anion (TcO4−), which only weakly adsorbs onto the surface of minerals and is consequently highly mobile in water. This presents considerable challenges in the management of nuclear fission waste. Indeed, 99Tc is the major source of activity, in becquerels (Bq) per mass, in spent nuclear fuel for the period from about 104−106 years. Paradoxically 99mTc decays to 99Tc in vivo, where the slow decay rate of the latter is seen as favorable, since it minimizes the dose to the patient. Technetium has also found use as a corrosion inhibitor in the steel industry and as an environmental water tracer.2,3 Technetium is the lightest element in the periodic table that lacks a stable isotope, and although gram quantities of 99Tc are available to researchers, it is not unusual to commence studies using Re as a nonactive surrogate.4−6 The validity of this has been questioned, given the difference in the standard redox potentials of Tc(VII) and Re(VII). There is an increasing body of work revealing significant dissimilarities between the © XXXX American Chemical Society

properties of Re and Tc oxides and of their speciation when they are incorporated in nuclear waste glass.7,8 Pertechnetates of the type ATcO4 (A = (NH4), K, Cs, Ag) were among the first Tc oxides isolated. In 1959 Boyd9 reported that the alkali-metal pertechnetate salts are isomorphous with the corresponding rhenium salts and that X-ray diffraction measurements had shown NH4TcO4, KTcO4, and AgTcO4 to have the CaWO4, or tetragonal scheelite, structure. Single-crystal X-ray diffraction studies in 1976, 1980, and 2003 for A = K, NH4, Ag, respectively, confirmed the structures to be in space group I41/a.10−12 Keller and Kanellakopulos13 reported that RbTcO4 is tetragonal, and this was confirmed in the recent powder neutron diffraction study of Weaver et al.14 Using powder XRD methods McDonald and Tyson15 showed the structure of CsTcO4 to be orthorhombic in Pnma, which was later confirmed using single-crystal methods by Meyer and Hoppe.16 Kanellakopulos reported that both CsTcO4 and TlTcO4 undergo a phase transition from an orthorhombic to tetragonal structure upon heating to above 220 °C,17 although details of the high-temperature structures have not been reported. CsTcO4 has received attention in recent years since the volatility of Tc during vitrification is significantly increased if Cs is present.18 The magnetic properties of the related Ru Received: April 30, 2019

A

DOI: 10.1021/acs.inorgchem.9b01257 Inorg. Chem. XXXX, XXX, XXX−XXX

T (K) space group a (Å) b (Å) c (Å) V (Å3) A x y z 100Uiso (Å2) Tc x y z 100Uiso (Å2) O1 x y z 100Uiso (Å2) O2 x y z 100Uiso (Å2) O3 x y z 100Uiso (Å2) 13.53076(14) 448.334(7) 0 0.25 0.625 1.82(5) 0 0.25 0.125 5.40(7) 0.0969(7) 0.0171(6) 0.19645(28) 5.27(19)

12.9206(2) 412.540(10)

0 0.25 0.625 1.12(6)

0 0.25 0.125 2.75(4)

0.1106(4) 0.0164(4) 0.19711(20) 1.46(12)

RbTcO4 295 I41/a 5.75625(5)

295 I41/a 5.65056(7)

KTcO4

0 0.5081(5) 0.68754(23) 22.31(24)

0 0.25 0.375 12.54(7)

0 0.75 0.125 5.99(5)

13.99553(10) 472.839(5)

673 I41/amd 5.81249(4)

RbTcO4

B

CsTcO4

TlTcO4

0.947(3) 1.0184(15) 0.3136(6) 6.5(5)

0.9943(17) 0.9881(16) 0.3112(6) 4.8(3)

0.180(8) 0.25 0.922(3) 19(2)

0.9714(8) 0.25 0.6214(5) 5.90(8)

0.0034(4) 0.25 0.12725(23) 4.96(3)

295 Pnma 5.49536(4) 5.74218(4) 13.46048(8) 424.750(5)

0.712(8) 0.25 0.9457(23) 21(2)

0.0429(14) 0.0273(8) 0.1881(4) 14.06(21)

0 0.25 0.125 10.00(5)

0 0.25 0.625 8.58(4)

14.61180(8) 520.714(4)

673 I41/a 5.96964(2)

0.6637(25) 0.25 0.9655(11) 6.7(6)

0.1767(22) 0.25 0.9162(8) 3.1(4)

0.9634(5) 0.25 0.62045(21) 5.30(8)

0.02685(33) 0.25 0.12546(15) 3.02(5)

295 Pnma 5.72402(4) 5.92067(4) 14.36516(10) 486.836(6)

CsTcO4

0.9521(26) 1.0106(12) 0.3218(5) 9.2(4)

0.7130(44) 0.25 0.9763(21) 34.6(13)

0.1800(25) 0.25 0.9089(9) 7.8(5)

0.9633(8) 0.25 0.62101(34) 8.07(8)

0.0163(4) 0.25 0.12624(17) 7.39(4)

470 Pnma 5.57970(4) 5.740059(28) 13.66612(7) 437.696(4)

TlTcO4

0 0.4837(10) 0.6764(6) 24.3(5)

0 0.25 0.375 10.23(8)

0 0.75 0.125 8.86(8)

13.84204(10) 455.902(5)

673 I41/amd 5.73900(4)

TlTcO4

Table 1. Selected Structural Parameters for the ATcO4 Oxides Obtained from Rietveld Refinements against Synchrotron X-ray Powder Diffraction Data AgTcO4

0.1210(4) −0.0018(5) 0.19951(19) 1.68(6)

0 0.25 0.125 1.579(13)

0 0.25 0.625 2.534(21)

11.86734(2) 335.5548(9)

295 I41/a 5.317469(7)

Inorganic Chemistry Article

DOI: 10.1021/acs.inorgchem.9b01257 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

al.12 (see Figure S1). In this structure, each Tc site is surrounded by four equivalent O sites with approximately tetrahedral symmetry and each Ag site is surrounded by eight O sites in a distorted-dodecahedral arrangement. The Tc−O distance is 1.730(2) Å, and the distortion of the tetrahedron is evident from the O−Tc−O angles, which are 105.24(12) and 118.3(2)°. The Ag cation has a distorted eight-coordinate geometry with two pairs of Ag−O distances, 4 × 2.546(2) and 4 × 2.567(2) Å, that are in reasonable agreement with the distances reported for isostructural AgReO4 (2.544(2) and 2.638(2) Å),26 although as noted by Sarsfield et al.12 the mean Ag−O distance is smaller in AgTcO4 than in AgReO4, suggesting that there is a strong interaction between the pertechnetate group and the silver cation. The tetragonal structure was retained upon heating to 748 K, above which the sample melted. The thermal expansion of the tetragonal cell of AgTcO4 between 293 and 748 K is anisotropic (see Figure S2) with the linear thermal expansion coefficients (TEC) defined as αi = (iTH − iTL)/(iTLΔT) ,where i is the unit cell direction, ΔT is the change in temperature, iTH is the lattice parameter in direction i at high temperature, and iTL is the corresponding lattice parameter at low temperature: αa = [23.7(3)] × 10−6 K−1 and αc = [43.6(6)] × 10−6 K−1. Similar anisotropy in the thermal expansion of other scheelites was described by Bayer, who concluded that the magnitude of the thermal expansion was enhanced by a large valence difference between the two cations.27 RbTcO4. At room temperature RbTcO4 also adopts a tetragonal scheelite-type structure in space group I41/a. The TcVII cation is bonded to four oxygen atoms with a Tc−O distance of 1.745(4) Å in a slightly distorted tetrahedron; the O−Tc−O angles are 107.88(12) and 112.7(2)°. The two Rb− O distances, 2.917(4) and 2.946(4) Å, are in good agreement with the distances reported for isostructural RbIO4 (2.911(2) and 2.979(2) Å28) and RbReO4 (2.924(6) and 2.995(6) Å29), providing further evidence that the bond lengths in AgTcO4 are anomalous. This tetragonal structure was retained on cooling the sample to 90 K, and there were no obvious differences in the diffraction profiles measured between 90 and 400 K, other than those associated with anisotropic thermal expansion. As illustrated by Figure 1, the thermal expansion between 90 and 400 K is highly anisotropic with the linear thermal expansion coefficients being αa = [−0.42(2)] × 10−6 K−1 and αc = [101(8)] × 10−6 K−1. The anisotropic thermal expansion in scheelite compounds was described by Bayer almost 50 years ago, who noted that scheelites invariably display higher thermal expansion in the c direction than in the a direction. This is believed to be a consequence of differences in the ordering of the two cations in the a and c directions, which results in a layered-type arrangement. Two-dimensional layered oxides are well-known to display much higher thermal expansion in the direction perpendicular to the layers.27 The near zero thermal expansion along the a axis is unusual and appears to be in response to the large positive thermal expansion of the c axis. The thermal expansion of the volume between 90 and 400 K is unexceptional. After the cryostream was replaced with a heater, patterns were collected on heating from 320 to 900 K; this results in a detectable change in the background intensity, as is evident in Figure 2. Around 500 K there is a marked change in the thermal expansion curves with αa increasing dramatically to 49.2 × 10−6 K−1 between 513 and 853 K and αc decreasing somewhat to 72.0 × 10−6 K−1. These values are close to that

and Os oxides that have a d1 electron configuration with the TM cation being a distorted tetrahedron are also of interest.19−21 The aim of the present work is to establish the thermal evolution of the crystal chemistry of some pertechnetates, with a focus on the phase transitions in the two orthorhombic oxides CsTcO4 and TlTcO4. We demonstrate an unusual order−disorder transition in TlTcO4 associated with the Tl+ 6s2 lone pair electrons and report for the first time the presence of a tetragonal−tetragonal phase transition in RbTcO4.

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EXPERIMENTAL SECTION

Caution! 99Tc is a β−emitter (Emax = 0.29 MeV). All operations relating to the synthesis of this sample were performed in a licensed radiochemical laboratory. Appropriate shielding was employed during all manipulations for loading of samples into containers for examination. Preparation. The ANO3 salts were purchased from Sigma-Aldrich and used as received. The NH4TcO4 salts were prepared and purified according to the method reported in the literature. The ATcO4 salts (A = Ag, Tl, Rb, Cs) were prepared at UNLV from the precipitation of NH4TcO4 solutions with ANO3 in deionized water. The salts were washed three times with deionized water and dried in a desiccator. Structural Studies. Synchrotron X-ray powder diffraction data were collected over the angular range 5 < 2θ < 85°, using X-rays of wavelength 0.60383 Å, as determined by structural refinement of NIST SRM660b LaB6 standard, on the powder diffractometer at beamline BL-10 of the Australian Synchrotron.22 Data were collected from 90 to 400 K using an Oxford Cryosystems Cryostream Plus and room temperature to 1000 °C using a Cyberstar hot-air blower. The cryostream and hot air blower were calibrated at the beamline using a combination of thermocouple measurements with a range of melting points and phase transition standards. Once these calibrations were applied, at the ramp rates used in this experiment, temperatures were accurate to within 1 K of the reported value. The samples were mixed with amorphous quartz powder (50 wt %) to minimize X-ray absorption and were housed in 0.5 diameter quartz capillaries that were rotated during the measurements. Structure refinements, using the Rietveld method, were carried out with the GSAS23 program and the EXPGUI24 front end. The peak shapes were modeled using a pseudo-Voigt function, and the background was estimated using an 18−21 term shifted Chebyschev function. The scale factor, detector zero point, lattice parameters, atomic coordinates, and atomic displacement parameters were refined together with the peak profile parameters.

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RESULTS AND DISCUSSION Synchrotron X-ray diffraction (S-XRD) was used to establish accurate and precise structures for the five pertechnetates ATcO4 (A = K, Rb, Cs, Tl, Ag) and to monitor the temperature dependence of their structures. The diffraction pattern for KTcO4 demonstrated the sample to contain a small amount of a second phase that was identified as (NH4)TcO4, and this phase was included in the Rietveld refinements.25 KTcO4 is isostructural with both KRuO4 and KOsO4.19,20 The structures for the three oxides with A = K, Rb, Cs refined against data measured at room temperature are in good agreement with those reported recently by Weaver et al.,14 who employed medium-resolution neutron diffraction. Likewise, the structure of AgTcO4 is in good agreement with the earlier single-crystal XRD study of Sarsfield et al.12 We are unaware of any contemporary studies of the structure of TlTcO4. The results of our studies are summarized in Table 1. Rietveld plots are given in the Supporting Information. AgTcO4. The diffraction pattern of AgTcO4 measured at room temperature was well fitted to the tetragonal scheelitetype structure in space group I41/a reported by Sarsfield et C

DOI: 10.1021/acs.inorgchem.9b01257 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 3. Portion of the observed S-XRD profiles for the hightemperature (600 K) I41/amd phase and low-temperature (400 K) I41/a phase of RbTcO4.

Figure 1. Temperature dependence of the lattice parameters for RbTcO4 from room temperature to 850 K. Where not evident, the estimated standard deviations (esds) are smaller than the symbols. The change in the thermal expansion evident near 450 K is due to the I41/a to I41/amd transition.

perrhenate.30 The I41/a to I41/amd transition is allowed, by group theory, to be continuous, and the diffraction data are consistent with this. Although Range et al.30 identified the I41/amd space group to be the higher symmetry scheelite aristotype, we are unaware of any previous reports of a temperature-induced I41/a to I41/amd transition in scheelite type oxides, although the pressure-induced I41/amd to I41/a transition in EuVO4 has been reported by Garg and Errandonea.31 Note that the ambient-pressure form of EuVO4 has the zircon structure, which is subtly different from that observed here, despite having the same space group. Above 450 K the tetragonal structure of RbTcO4 is described by space group I41/amd. The TcVII cation is bonded to four oxygen atoms with a Tc−O distance of 1.648(3) Å in a distorted tetrahedron; the O−Tc−O angles are 106.51(11) and 115.6(2)°. The Tc−O distance is actually shorter than the value seen at room temperature, whereas the two Rb−O distances, 3.056(4) × 4 and 3.411(2) × 8 Å, are noticeably longer. The lengthening of the Rb−O distances reflects the increase in the effective coordination number of the Rb from 8 to 12. CsTcO4. As described by Weaver, CsTcO4 has an orthorhombic structure in space group Pnma at room temperature.14 This structure is sometimes referred to as the pseudo-scheelite structure to highlight the similarities of the structural motifs (see Figure 4) and is favored as the size of the A-site cation increases. In this structure, the TcVII cation is bonded to four oxygens in a distorted tetrahedron with Tc−O distances of Tc−O(1) = 1.707(13) Å, Tc−O(2) = 1.671 Å, and Tc−O(3) = 2 × 1.757(9) Å for an average Tc−O distance of 1.71 Å. The Cs−O distances vary between 3.089(8) and 3.686(8) Å. Cooling the sample to 90 K resulted in a noticeable increase in the magnitude of the orthorhombic distortion (see Figure 5 and Figure S3). As is apparent from these figures, the orthorhombic distortion disappears around 430 K and the S-XRD profiles measured above this temperature could be fitted to tetragonal models in either I41/a or I41/amd. The transition to the tetragonal structure is accompanied by a discontinuous jump in both the c lattice parameter and the cell volume. Scrutiny of the profiles measured between 400 and 450 K showed evidence for the coexistence of the tetragonal and orthorhombic phases indicative of a first-order phase transition (see Figure 6).

Figure 2. Contour plot of the temperature dependence of a portion of the XRD patterns for RbTcO4 during heating. The change in background around 400 K is due to the change from a cryostream to a heater. The loss of diffraction above ∼830 K is due to sample melting. This figure illustrates both the change in anisotropic thermal expansion around 500 K and the loss of the 114 reflection at the same temperature indicative of the presence of the temperatureinduced I41/a to I41/amd phase transition.

seen for AgTcO4. Examination of the diffraction profiles shows this is a consequence of a transition from the tetragonal I41/a structure to a second tetragonal structure in I41/amd. These two structures can be distinguished by the extinction of reflections such as the 114 (2h + l ≠ 4n). The (114) reflection has a maximum intensity of ∼0.9% of the strongest (112) reflection: compare Figure 3 and Figure S1. As illustrated in Figure 2, the loss of observable intensity at the (114) reflection occurs at the same temperature as the change in thermal expansion. In I41/a the x parameter for the oxygen in the general position 16f is a variable, whereas this is constrained to x = 0 for the equivalent 16h position in I41/amd. The two tetragonal structures differ in the nature of the orientation of the TcO4 tetrahedra, as illustrated in Figure 4. The hightemperature form of RbTcO4 is thus isostructural with αCsReO4, the high-temperature modification of cesium D

DOI: 10.1021/acs.inorgchem.9b01257 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. Representations of the (a) I41/a, (b) I41/amd, and (c) Pnma structures observed for the ATcO4 oxides. The I41/a structure can be viewed as being obtained from the I41/amd structure by rotation of the TcO4 tetrahedra about the b axis. The Pnma structure involves rotation of the TcO4 tetrahedra along the c axis. In all cases the Tc cations are at the center of the polyhedra; the A-type cations are represented by the large gray spheres and the oxygen atoms by the small red spheres. In both the I41/a and I41/amd structures there is only a single type of oxygen atom.

Figure 6. Portions of the observed and calculated Rietveld profiles for CsTcO4. The upper profile measured at 450 K was fitted using a tetragonal I41/a model, and the lower profile measured at 430 K was fitted using an orthorhombic Pnma model. At 440 K these two phases coexist.

Figure 5. Temperature dependence of the lattice parameters for CsTcO4 from room temperature to 850 K. Where not evident, the esds are smaller than the symbols. The figure highlights the change in the thermal expansion associated with the Pnma to I41/a phase transition near 430 K.

positions are essentially degenerate and so, unlike the case for RbTcO4 described above, the extinction of the former cannot be established. Since the transition is first order and a Pnma to I41/amd transition is allowed to be continuous, whereas a Pnma to I41/a transition must be first order, and Ox is statistically not equal to zero, we tentatively postulate that, for the high-temperature tetragonal structure, the appropriate space group is I41/a and the structures between 440 and 850 K were refined on this basis. Range and co-workers described an orthorhombic (Pnma) to tetragonal transition in the analogous Re oxide CsReO4 and concluded that the tetragonal structure was in I41/amd and on the basis of DSC measurements suggested that the transition was first order.30 There is merit in confirming the order of this transition. The Pnma to I41/a transition in CsTcO4 occurs at around 440 K. This temperature is comparable to that observed in CsReO4, where different authors report the transition as

Establishing the space group of the high-temperature phase was problematic. Models in both I41/a and I41/amd were tested, and these gave identical measures of fit χ2 = 6.4 and Rp = 0.075. The former model gave Ox = 0.0390(26) and Uiso(O) = 0.194(4) Å2, whereas for the latter Ox is required to be equal to zero and Uiso(O) was refined to be 0.199(4) Å2. Although Ox is, within the precision of the refinements, statistically greater than zero, that an identical quality fit is obtained with this constrained to be equal to zero demonstrates the insensitivity of the data to this. This is partially due to the large contribution of the heavy Cs and Tc cations to the diffraction data and is further affected by the large thermal displacements of the anion. A further complication is that over the entire tetragonal region the (114) and (202) reflection E

DOI: 10.1021/acs.inorgchem.9b01257 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry occurring between 405 and 455 K.17,30,32 The nonlinear expansion in the c axis, evident in Figure 5, drives the changes in the c/a ratio, and it is possible that this is related to the proximity of the melting point of the sample. In the high-temperature tetragonal structure, the TcVII cation is bonded to four oxygen atoms with a Tc−O distance of 1.625(10) Å (at 873 K) in a distorted tetrahedron; the O− Tc−O angles are 131.2(1) and 99.8(5)°. The Cs remains coordinated to eight oxygen atoms with four at 3.103(12) and four at 3.457(11) Å. There are another four contacts to oxygen atoms at a distance of 3.881(15) Å. These are effectively nonbonding, since bond valence sum (BVS) calculations show each contributes only 0.02 valence unit. The temperature dependence of the Tc−O bond distances is illustrated in Figure 7 and that of the Cs−O distances are

pertechnetate cation. The Cs−O distances increase with increasing temperature in both the Tc and Re oxides.30 TlTcO4. S-XRD profiles for a single sample of TlTcO4 were measured at 10 K intervals from 90 to 400 K and then at various intervals from 320 to 800 K, above which the sample melted. To cover this temperature range, it was necessary to change the sample environment, which required returning the sample to ambient temperature for an extended period. A contour plot of the temperature dependence of the XRD patterns for TlTcO4 during heating is shown in Figure 8.

Figure 8. Contour plot of the temperature dependence of the XRD patterns for TlTcO4 during heating. The change in background around 400 K is due to the change from a cryostream to a heater. The loss of diffraction above ∼810 K is due to sample melting. This figure illustrates the presence of two temperature-induced phase transitions, near 380 and 490 K.

Figure 7. Temperature dependence of the Tc−O bond distances in CsTcO4. The structures were refined in space group I41/a above 420 K and in Pnma below this. The dashed lines indicate the positions of the structural phase transitions.

Features of note in this figure are the presence of two temperature-induced phase transitions, near 380 and 490 K, and the loss of diffraction above ∼810 K due to sample melting. The change in background around 400 K, evident in Figure 8, is due to the change from a cryostream to a heater. The S-XRD profile for TlTcO4 measured at 90 K was well fitted to a model in the orthorhombic space group Pnma. Heating to 370 K resulted in no significant changes in the diffraction profiles, other than those associated with anisotropic thermal expansion. Further heating to 380 K resulted in splitting of a small number of peaks in the diffraction pattern. This splitting vanished as the heating was continued to 390 K, at which temperature the profile was well fitted to the same orthorhombic model in SG Pnma used for the 90 K analysis. After the cryostream was replaced with a heater, patterns were then collected on heating from 320 to 800 K. A dramatic change in the profiles was apparent at 500 K, and analysis of the pattern demonstrated the structure to be tetragonal above 500 K. Further examination of the profile showed no evidence for the (114) reflection, demonstrating the tetragonal structure to be in space group I41/amd rather than I41/a. The measures of fit in the former were also superior: χ2 = 3.5 and Rp = 0.064 vs χ2 = 4.0 and Rp = 0.068, respectively. The temperature dependence of the lattice parameters obtained by Rietveld analysis is shown in Figure 9. This figure emphasizes the discontinuous nature of the phase transition near 380 K. Figure 9 reveals that as the a axis of the orthorhombic phase decreases discontinuously near 390 K, the c axis increases abruptly. As noted above, the S-XRD data measured at 380 K

shown in the Supporting Information. While there is some scatter in the derived distances, it is apparent that there is no significant change in the Tc−O or Cs−O distances throughout the orthorhombic phase; this observation is significant in considering the behavior of TlTcO4 described below. Figure 7 reveals an unusual minimum in the Tc−O distance near 400 K that accompanies the Pnma to I41/a transition. This suggests considerable overbonding of the Tc cations in the tetragonal structure; BVS calculations give an effective valence of 9.2, in comparison to 7.0 in the ideal case. In the orthorhombic structure the BVS of the Tc is 5.8. The BVS of the Cs cation is effectively the same in both structures, ∼0.9, and it appears that the transition is driven by a need to relieve the poor bonding environment of the small, highly charged Tc cation. Considering the thermal expansion, Figure 5 shows an unusual minimum in the c/a ratio near 725 K. This is not reflected in changes in either the Tc−O or Cs−O bond distances that evolve continuously through this region. The nonlinear expansion in the c axis, evident in Figure 5, drives the changes in the c/a ratio, and it is possible that this is related to the proximity of the melting point of the sample. The average Tc− O distance is shorter in the HT tetragonal structure than in the LT orthorhombic structure. Similar behavior is seen in CsReO4 where the average Re−O distance in the HT tetragonal structure, 1.68 Å, is shorter than the 1.71 Å seen in the orthorhombic structure. 33 This highlights the importance of the interaction of the A-site cation and the F

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reminiscent of the fergusonite (SG I2/a) to scheelite (I41/a) transition in LaNbO4.33,35 As in LaNbO4 the evolution of the lattice parameters of TlTcO4 approaching the transition to the higher symmetry tetragonal structure are suggestive of a continuous phase transition; however, the coexistence of the two phases (I2/a and I41/a in LaNbO4 compared with Pnma and I41/amd in TlTcO4) shows the transition must be first order. That the Pnma and I41/amd phases coexist is illustrated in Figure 11. In both space groups the structure consists of

Figure 9. Temperature dependence of the lattice parameters for TlTcO4 estimated by Rietveld refinement of S-XRD data. Figure 11. Portions of the Rietveld profiles for TlTcO4 around the Pnma to I41/amd transition. The lower set of tick marks in the pattern at 490 K is from the Pnma phase.

showed some peak splitting that could not be modeled using a single-phase Pnma model. Attempts to fit this using the monoclinic structure seen in TlReO434 were unsuccessful, suggesting that the observed peak splitting was not due to symmetry lowering. Since models in Pnma fitted the S-XRD profiles measured below 300 K and between 400 and 500 K, albeit with noticeably different unit cell parameters, a twophase model with both phases in Pnma was developed and this provided a good fit to the data measured at 380 K. The coexistence of the two phases suggests that the transition between them must be first order, and this is demonstrated by the observed hysteresis. The profile measured at 320 K (∼50 °C) is well fitted with a single orthorhombic phase, but that measured after cycling the sample to 400 K and then returning it to room temperature before reheating to 320 K clearly contains two phases (see Figure 10). Figure 9 illustrates the rapid change in the a and b axis associated with the Pnma−I41/amd transition. This behavior is

isolated TcO4 tetrahedra which are linked by thallium cations. In the high-temperature tetragonal structure, the thallium cations are surrounded by 12 oxygen atoms at distances of 2.960(4) Å × 4 and 3.2108(21) Å × 8 with an average Tl−O distance of 3.127 Å (values at 500 K), corresponding to an effective bond valence of the Tl cation of 0.84. The TcO4 tetrahedra are flattened along the c axis, and the O−Tc−O angles differ from the ideal value of 109°, being 100.55(15) and 129.3(4)°. The Tc−O distance within the tetrahedra is 1.730(4) Å with the BVS being 6.65. Figure 11 also reveals that the peaks of the orthorhombic phase are broadened relative to those from the tetragonal phase. Peak broadening can occur as a consequence of microstrain and/or size effects. In the present case, where the sample undergoes a first-order phase transition between the orthorhombic and tetragonal structures, it is likely that, when domains of the minority orthorhombic phase exist, these will be small and perhaps subject to microstrains. While the tetragonal to orthorhombic transition in TlTcO4 is unexceptional and can be explained by the anisotropic thermal expansion stabilizing the tetragonal structure, the observation of the first-order orthorhombic to orthorhombic transition is unexpected. Isosymmetric phase transitions in systems containing transition metals generally occur due to coupling of the electronic or magnetic degrees of freedom of the metal with the lattice. This is well illustrated in the orbital ordering in LaMnO3 and substituted variants.36 LaMnO3 undergoes transformation from the JT distorted orthorhombic phase (O′) to the high-temperature orthorhombic phase (O), which is nearly cubic. Both phases are described by the same space group Pbnm. TcVII is nonmagnetic (4d0 electron configuration), thus ruling out a magnetic contribution to the transition. The Tl+ cation has a 6s2 electron configuration, and it is possible that the 6s lone pair electrons drive the transition. Cations with a ns2 lone pair configuration, including Tl+, Pb2+, Bi3+, Sn2+, and Sb3+, can exhibit instabilities as a

Figure 10. Portion of the Rietveld profiles for TlTcO4. The top profile was recorded as the sample was heated from 90 to 400 K, and the lower profile was recorded after cooling the sample to room temperature and then reheating it to 320 K. In both cases the solid lines are the results of the Rietveld fitting. G

DOI: 10.1021/acs.inorgchem.9b01257 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry consequence of a pseudo-Jahn−Teller effect. This can be manifested as a ferroelectric instability where the lone pair stereochemical activity results in coherent cation displacement: for example, in Pb(Zr1−xTix)O3 and [SbCl5]2−. There is mounting evidence from a number of structural families including chalcogenides such as SnTe or PbS,37 halide perovskites such as CsSnBr3,38 and pyrochlore oxides such as Bi1.5Zn0.5(Nb1.5Zn0.5)O739 that the cations can be displaced in a temporally incoherent fashion. In PbS, for example the Pb 6s2 lone pair electrons are frozen and only become stereochemically active at higher temperatures, 40 whereas in [H 2 dmdap][SbCl5] (dmdap = N,N-dimethyl-1,3-diaminopropane), the stereochemical activity emerges at low temperatures.41 There are a number of examples of Tl+ oxides where the 6s2 electrons are stereochemically active, which invariably leads to a lengthening of the Tl−O bond. This is illustrated in the thallium borate TlB3O5, where the Tl−O distances in the TlO6 polyhedra range between 2.662 and 3.198 Å (Δl = 0.536 Å),42 whereas in the analogous Cs oxide CsB3O5, the Cs−O distances, while longer, reflecting the increase in the ionic radius of Cs+, have a much smaller range, between 3.030 and 3.342 Å, with Δl = 0.312 Å.43 Comparison of the two orthorhombic structures appears to rule out the 6s2 electrons being coherently displaced. In the low-temperature Pnma structure the Tl cation is effectively eight-coordinate with a wide range of Tl−O distances ranging from 2.50(3) to 3.138(9) Å. The remaining four Tl−O contacts given in Table 2 at 3.531(10) and 3.609(17) Å are

Figure 12. Temperature dependence of the Tl−O bond distances in TlTcO4.

450 K, resulting in the sequence of structures I41/a → P21/n → I41/a. There are three Tl sites in the monoclinic structure, two of which are eight-coordinate and the other is ninecoordinate (considering Tl−O distances