Abstract—The isobaric heat capacities of two monoclinic (M’ and M) modifications of yttrium orthotantalate at temperatures 5–1300 K have been measured by the adiabatic and differential scanning calorimetry methods. It has been demonstrated that the difference in structure between the crystal lattices of M’ and M has little effect in the heat capacity, and the difference between the heat capacities of these phases Cp(M) – Cp(M’) is small, always positive, and increases in the range of the lowest temperatures. The unit cell parameters of MYTaO4
have been determined as a function of temperature in the range 300–1173 K.

Rare earth orthotantalates are characterized by high density, chemical stability, and biological inertness. This, along with unique physicochemical properties, makes orthotantalates promising for creating functional materials. Increased attention to rare earth orthotantalates and, in particular, to M-YTaO4 is associated with the possibility of their use as the basis of thermal barrier materials for gas turbine power plants, as well as stabilizing additives to zirconium dioxide [1, 2]. Yttrium orthotantalate crystalizes in the monoclinic system M (fergussonite) and reversibly transforms to the high-temperature tetragonal modification (scheelite) T at ~1700 K [3]. Common methods of synthesis of complex oxides, for example, sol-gel or coprecipitation from solutions with subsequent annealing, lead to the monoclinic orthotantalate M’, and sometimes, to a more symmetric form—tetragonal orthotantalate T’ [4]. Heating modification T’ leads to irreversible transformation of yttrium orthotantalate to monoclinic form M’, which is quite stable up to ~1700 K; however, further heating leads to the irreversible transition to monoclinic modification M with doubled parameter b and, hence, the doubled number of formula units. An analogous situation was observed for gadolinium orthotantalate [5]; the coexistence of two monoclinic modifications M and M’ was detected, so that, among two possible schemes of structural transformation

М’ →T ↔ M,                                                                 (1)
М’ → М + M’ → M ↔T.                                                        (2)

The second one is preferable. In any case, no reverse transformation into M’ is observed on cooling. This allows us to consider that the M’ modification is metastable. Since the M’ modification is stable up to high temperatures (~1700 K), it was interesting to determine the effect of structural differences on the isobaric heat capacity of these phases. Previously [6], we have reported the results of measuring the lowtemperature heat capacity of M yttrium orthotantalate; the present study focuses on measuring the lowtemperature heat capacity of M-YTaO4 by the adiabatic calorimetry method (5–350 K) and the hightemperature heat capacity of M’- and M-YTaO4 by the differential scanning calorimetry (DSC) method (330–1400 K). Like M’-YTaO4;, stable M-YTaO4 was synthesized by back precipitation with subsequent annealing at higher temperatures than M’-YTaO4. Unit cell parameters determined by X-ray powder diffraction at room temperature were a = 5.293(1) Å, b =5.445(1) Å, c = 5.108(1) Å, β = 96.45(2)°, V3 = 146.23 Å3 for M’; a = 5.325(2) Å, b = 10.934(2) Å, c = 5.051(1) Å, β = 95.19(4)°, V3 = 298.88 Å3. These values are consistent with the data in [7, 8]. Figure 1 shows the difference between the isobaric low-temperature heat capacities of the monoclinic phases Cp(M) – Cp(M’). This difference in heat capacity values is small; owever, it falls outside the limits of the confidence interval and reaches the highest relative values in the range of the lowest temperatures. Measurements at high temperatures (330–1400 K) made it possible to describe the isobaric heat capacity in the generally accepted form by the Maier–Kelley equation:


Fig. 1. Difference between the low-temperature heat capacities of M- and M’-YTaO4.

Cp(M), J/(mol K) = 141.89 + 0.011588T — 2663694/T2                                (3)

Cp(M), J/(mol K) = 135.75 + 0.010951T — 1942740/T2.                         (4)

The temperature dependence of the heat capacity difference is

Cp(M) — Cp(M), J/(mol K) = 6.14 + 0.0006337T — 720954/T2                  (5)

the error of the DSC method. Nevertheless, it can be argued that the high-temperature heat capacity of the stable phase is somewhat higher, as well as in the case
of low temperatures, despite the higher density of the M’ modification. wholesale custom jerseys It should be noted that there are no anomalies in the temperature dependences of the heat
capacities for both phases in the temperature range 5–1400 K. Since the literature contains information on the use of yttrium tantalate as a high-temperature material, it
was of interest to determine the effect of heating on the monoclinic unit cell parameters. Temperature dependences of the unit cell parameters
of the stable monoclinic phase were measured on a Shimadzu diffractometer with an HA-1001 accessory for high-temperature measurements in the range
300–900 K. Figure 2 shows the temperature dependence of the unit cell parameters in comparison with the data from [9]. It can be seen that there is complete coincidence. Increasing temperature leads to a decrease in parameter a and an increase in parameter c until their complete equalization as the temperature of the transformation of YTaO4 to the tetragonal modification T is reached. At the same time, an almost linear increase in the parameter b is observed. The analytical form of the temperature dependences of the unit cell parameters of M-YTaO4, as well as the unit cell volume (Z = 4), is given in Table 1 for the totality of our values and the data of [9] in the form of polynomials ∑ni=0aiTi. It can be seen from Fig. 2 that the phase transition from the M to T phase proceeds gradually without a jump-like change of the lattice parameters at the transformation point, similarly to secondorder transitions. This suggests that the process of cyclic temperature changes will not destroy the integrity of the thermal barrier coating.

Table 1. Unit cell parameters of M-YTaO4 as a function of temperature (300–1600 K), ∑ni=0aiTi

a a(T), Å b(T), Å c(T), Å V3, Å/300–900 K
a0 5.32938 ± 0.02484 5.45261 ± 0.00284 5.07094 ± 0.02059 290.960 ± 0.297
a1 –(6.67343 ± 1.66613) × 10–4 (4.25927 ± 0.60063) × 10–5 –(1.59992 ± 1.08328) × 10–4 (57.500 ± 8.936) × 10–4
a2 (2.68156 ± 4.03443) × 10–7 (1.07101 ± 0.28998) × 10–8 (4.16642 ± 1.89733) × 10–7 (1.9031 ± 0.5987) × 10–7
a3 –(3.89103 ± 4.50765) × 10–10 –(3.46339 ± 1.35016) × 10–10
a4 (2.72846 ± 2.35949) × 10–13 (1.08963 ± 0.33577) × 10–13
a4 –(7.70815 ± 4.68051) × 10–17
R2 0.99653 0.9953 0.9969 0.9993


Fig. 2. Temperature dependences of the unit cell parametersof M-YTaO4: (1) data of [9] and (2, 3) this work.

 

ACKNOWLEDGMENTS
This work was supported by the Russian Science Foundation (project no. 18–13–00025) and performed with the use of equipment of the Shared Facility Center, Institute of General and Inorganic Chemistry, RAS.
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Translated by G. Kirakosyan

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