Crystallography

B. P. Sobolev, D. A. Mineev, V. P. Pashutin

On the Low-Temperature Hexagonal Modification of NaYF₄ with the Gagarinite Structure

(Presented by Academician N. V. Belov, 17 XII 1962)

The question of the structure of the low-temperature modification of the compound with the stoichiometric formula NaF·YF₃ = NaYF₄ has until recently remained open. There are indications (³, ⁴) that upon annealing and synthesis of the high-temperature modification NaYF₄, which has the fluorite structure, a phase is obtained whose powder X-ray pattern cannot be indexed on the basis of a cubic cell. More detailed data on the structure of NaYF₄ are not given.

For the purpose of further study of the compound NaYF₄, the latter was obtained by us by two methods.

Table 1

Results of the calculation of the powder X-ray pattern of synthetic β-NaYF₄ and gagarinite

Fig. 1. Diffraction patterns: a — product of hydrothermal recrystallization at 500° (49 h) of the alloy NaYF₄; b — product of hydrothermal synthesis from NaF and Y₂O₃ at 500° (18 h); c — natural gagarinite Na(CaY)F₆. Recording conditions: URS-50I, Fe anode, b/f, 7 mA, 25 kV.

1) By recrystallization of a melt of stoichiometric composition NaYF₄ under hydrothermal conditions at a temperature below the transition to the α → β modification. As we were able to verify, this method is much more effective than annealing, especially at such temperatures as 500° (the upper limit was set by the temperature of the α → β transition).

2) By synthesis of NaYF₄ under hydrothermal conditions from sodium fluoride and yttrium oxide (molar ratio in the mixture Na : Y = 1 : 1). To fluorinate the yttrium oxide, the autoclave was filled with a 10% solution of hydrofluoric acid (degree of filling 0.3). The duration of the experiments was 18 h at 500°.

Identification of the products obtained under the above conditions was carried out by X-ray and chemical analyses. The X-ray study was performed on a URS-50I diffractometer. Fe $K_\alpha$ radiation was used; reflection recording from flat specimens; recording conditions 7 mA at 25 kV. Lines belonging to β-radiation were filtered out after measurement and are not included in the tables. In addition, powder patterns were obtained with an RKD-57 camera, copper radiation, Ni filter.

Table 2

Chemical analysis of the synthesis product*

* Analyst A. V. Bykova.

The results of the X-ray investigation are given in Table 1 and in Fig. 1. The products obtained by both the first and the second methods proved to be completely identical and have a structure sharply different from the high-temperature (fluorite) α-NaYF₄ modification.

Recalculation of the analytical results given in Table 2 gives the formula

[
\mathrm{Na}{1.04}(\mathrm{Y}}\mathrm{Fe}^{2+{0.03}\mathrm{Ca}){1.00}\mathrm{F}
\quad \text{or} \quad
\mathrm{NaYF}_4.
]

Analyzing the X-ray data for β-NaYF₄ presented above, we came to the conclusion that they coincide completely with the X-ray characteristics of gagarinite—a recently discovered mineral with the formula Na(YCa)F₆ (¹, ²). This similarity follows from the diffractograms shown in Fig. 1, and is also clearly observed in comparison of ordinary powder X-ray patterns. To determine the unit-cell parameters of β-NaYF₄, we indexed the powder pattern of this compound (see Table 1). The lattice parameter $c$ proved to be equal to the corresponding value for natural gagarinite, while the parameter $a$ is somewhat smaller.

Fig. 2. Heating curves: a—synthesized Na(Y₁․₅Na₀․₅)F₆; b—natural gagarinite Na(YCa)F₆.

Thus, we have shown that the low-temperature modification β-NaYF₄ crystallizes in the gagarinite structure with parameters of the hexagonal unit cell $a_0 = 5.96$ Å, $c_0 = 3.53$ Å. For natural gagarinite, we have (¹) $a = 5.99$ Å, $c = 3.53$ Å.

The calculation of the number of molecules per unit cell (based on the composition $\mathrm{NaYF}4$ and taking the density of the low-temperature modification of this compound to be $4.23\ \mathrm{g/cm^3}$ ($^3$)) gives 1.47 formula units. This value corresponds to the gross formula $\mathrm{Na}}\mathrm{Y{1.5}\mathrm{F}_6$, i.e., to the stoichiometry of gagarinite, $\mathrm{Na}(\mathrm{YCa})\mathrm{F}_6$. Taking into account the crystallochemical studies of the latter ($^2$), the formula $\mathrm{NaYF}_4$ should be written in the form $\mathrm{Na}(\mathrm{Y}_6$. Such a notation will fully reflect the crystallochemical features of the $\beta$-modification of this compound.}\mathrm{Na}_{0.5})\mathrm{F

To determine the temperature of the $\beta \to \alpha$ modification transition, we carried out thermographic analysis of the synthesized samples. The heating curve of one of them is shown in Fig. 2. The endothermic effect at $670^\circ$ (temperature measured from the onset of the effect) corresponds to the transition of the hexagonal modification into the cubic one. A similar effect on the heating curve of natural gagarinite begins at a temperature of $685^\circ$. The reverse transition ($\alpha \to \beta$) is considerably hindered, as a result of which the $\beta$-modification can be obtained only by prolonged annealing.

X-ray diffraction studies of the product heated above $670^\circ$ showed that it indeed has the fluorite structure indicated earlier for the $\alpha$-modification $\mathrm{Na}(\mathrm{Y}{1.5}\mathrm{Na}_6$ ($^4$).})\mathrm{F

The authors express their gratitude to Yu. A. Pyatenko for his attention to this work and for valuable comments during discussion of the manuscript.

Institute of Mineralogy, Geochemistry, and Crystallochemistry
of Rare Elements

Received
6 XI 1962

CITED LITERATURE

$^1$ A. V. Stepanov, E. A. Severov, DAN, 141, No. 4, 954 (1961).
$^2$ A. A. Voronkov, N. G. Shumyatskaya, Yu. A. Pyatenko, Zhurn. strukturn. khim., No. 1 (1963).
$^3$ W. Nowacki, Zs. Kryst., A, 100, No. 3, 242 (1938).
$^4$ F. Hund, Zs. anorg. Chem., 261, H. 1—2, 106 (1950).
$^5$ V. I. Mikheev, X-ray Determinative Tables of Minerals, Moscow, 1957.