New examples of non-cooperative octahedral tilting in a double perovskite: phase transitions in K₃GaF₆

The crystal structures of three polymorphs of K₃GaF₆ and the transition temperatures between these phases are reported for the first time. Synchrotron powder diffraction data clearly show that at 300 K α-K₃GaF₆ crystallizes in space group I4₁/a with lattice parameters of a = 19.1124 (3) Å, c = 34.4165 (6) Å, and Z = 80. The structure is based on the double perovskite but with two fifths of the GaF₆ octahedra rotated by ∼45°. This phase remains stable until ∼460 K, above which it undergoes a transition to I4/m with lattice parameters of a = 13.6088 (4) Å, c = 8.6764 (3) Å, and Z = 10 at 485 K. β-K₃GaF₆ has a similar structure but with only one fifth of the GaF₆ rotated by ∼45°. Above ∼510 K, the cubic Fm3m δ-K₃GaF₆ structure is stabilized, with a lattice parameter of a = 8.6649 (1) Å at 550 K. The F atoms have highly anisotropic displacement parameters which suggest dynamic octahedral tilting is occurring. This work expands the fairly small group of double perovskite compounds which display non-cooperative patterns of octahedral tilting.

Computing details

(K3GaF6_alpha)

Crystal data

F120Ga20K60	c = 34.4165 (6) Å
Mr = 6020.04	V = 12573.2 (4) Å^3
Tetragonal, I41/a	Z = 4
a = 19.1124 (3) Å	Dx = 3.180 Mg m^−3

Data collection

h = →	l = →
k = →

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
K1	0.00000	0.25000	0.3651 (11)	0.0171 (14)*
K2	0.8898 (12)	0.4468 (14)	0.5118 (8)	0.0171*
K3	0.1961 (14)	0.3217 (13)	0.4966 (7)	0.0171*
K4	0.0861 (10)	0.5600 (11)	0.3780 (7)	0.0171*
K5	0.2035 (13)	0.3246 (14)	0.3749 (8)	0.0171*
K6	0.00000	0.25000	0.5040 (10)	0.0171*
K7	0.9003 (11)	0.4539 (11)	0.3649 (7)	0.0171*
K8	0.9280 (10)	0.0938 (12)	0.3212 (6)	0.016 (2)*
K9	0.8680 (13)	0.3237 (12)	0.4340 (8)	0.0164*
K10	0.00000	0.25000	0.62500	0.0171*
K11	0.6912 (14)	0.8355 (12)	0.7448 (7)	0.0171*
K12	0.1663 (14)	0.6827 (13)	0.3051 (7)	0.0164*
K13	0.7263 (12)	0.5110 (15)	0.3113 (10)	0.0164*
K14	0.1023 (14)	0.0447 (14)	0.2545 (7)	0.0171*
K15	0.00000	0.25000	−0.0063 (10)	0.0171*
K16	0.0958 (13)	0.0480 (12)	0.1182 (7)	0.0171*
K17	0.00000	0.25000	0.12500	0.0171*
K18	0.0733 (10)	0.9058 (12)	0.9359 (9)	0.0164*
Ga19	0.0543 (8)	0.3990 (7)	0.4367 (4)	0.0155 (6)*
Ga20	0.6493 (8)	0.6983 (7)	0.8122 (5)	0.0155*
Ga21	0.5502 (7)	0.8987 (7)	0.1882 (5)	0.0155*
Ga22	0.2474 (8)	0.5008 (7)	0.0618 (5)	0.0155*
Ga23	0.5483 (6)	0.8979 (7)	0.6874 (4)	0.0155*
F1	0.107 (2)	0.425 (3)	0.4788 (12)	0.0150 (15)*
F2	−0.017 (2)	0.378 (3)	0.3967 (12)	0.0150*
F3	0.736 (2)	0.740 (2)	0.8148 (17)	0.0150*
F4	0.555 (2)	0.649 (2)	0.8112 (15)	0.0150*
F5	0.636 (2)	0.937 (2)	0.1814 (15)	0.0150*
F6	0.022 (2)	0.4921 (19)	0.4381 (17)	0.0150*
F7	0.098 (3)	0.315 (2)	0.4299 (15)	0.0150*
F8	0.606 (3)	0.784 (2)	0.8145 (17)	0.0150*
F9	0.687 (2)	0.6064 (19)	0.8127 (17)	0.0150*
F10	0.505 (2)	0.985 (2)	0.1863 (19)	0.0150*
F11	0.554 (3)	0.912 (3)	0.2413 (11)	0.0150*
F12	−0.019 (2)	0.371 (3)	0.4743 (13)	0.0150*
F13	0.120 (2)	0.426 (2)	0.3959 (12)	0.0150*
F14	0.651 (3)	0.696 (3)	0.8675 (11)	0.0150*
F15	0.660 (3)	0.692 (3)	0.7582 (11)	0.0150*
F16	0.468 (2)	0.848 (2)	0.1861 (15)	0.0150*
F17	0.598 (2)	0.814 (2)	0.1867 (17)	0.0150*
F18	0.549 (3)	0.889 (3)	0.1340 (10)	0.0150*
F19	0.279 (2)	0.5946 (19)	0.0640 (16)	0.0150*
F20	0.162 (2)	0.533 (2)	0.0623 (16)	0.0150*
F21	0.2579 (19)	0.501 (3)	0.1156 (10)	0.0150*
F22	0.225 (2)	0.402 (2)	0.0684 (13)	0.0150*
F23	0.337 (2)	0.467 (2)	0.0626 (15)	0.0150*
F24	0.261 (2)	0.495 (3)	0.0092 (10)	0.0150*
F25	0.629 (2)	0.956 (3)	0.6879 (19)	0.0150*
F26	0.496 (2)	0.982 (2)	0.6918 (17)	0.0150*
F27	0.562 (3)	0.901 (3)	0.7461 (10)	0.0150*
F28	0.468 (2)	0.854 (3)	0.7054 (12)	0.0150*
F29	0.600 (2)	0.812 (2)	0.6869 (16)	0.0150*
F30	0.526 (2)	0.894 (3)	0.6360 (11)	0.0150*

(K3GaF6_I4_m)

Crystal data

F30Ga5K15	c = 8.6764 (3) Å
Mr = 1505.01	V = 1606.9 (1) Å^3
Tetragonal, I4/m	Z = 2
a = 13.6088 (4) Å	Dx = 3.111 Mg m^−3

Data collection

h = →	l = →
k = →

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
K1	0.8033 (4)	0.0972 (4)	0.2500	0.0711 (6)*
K2	0.5000	0.0000	0.7500	0.0711*
K3	0.0000	0.0000	0.5000	0.0757 (11)*
K4	0.9114 (7)	0.2745 (7)	0.0000	0.0757*
Ga1	0.0000	0.0000	0.0000	0.0203 (4)*
Ga2	0.6996 (3)	0.9003 (2)	0.5000	0.0203*
F1	0.0545 (18)	0.1291 (12)	0.0000	0.0468 (9)*
F2	0.8443 (19)	0.4690 (9)	0.0000	0.0468*
F3	0.1767 (9)	0.0473 (16)	0.5000	0.0468*
F4	0.3144 (8)	0.0651 (7)	0.7089 (6)	0.0468*
F5	0.7594 (19)	0.7740 (9)	0.5000	0.0468*
F6	0.8566 (16)	0.4264 (9)	0.5000	0.0468*
F7	0.0000	0.0000	0.2089 (6)	0.0468*

(K3GaF6_Fm-3m)

Crystal data

F6GaK3	V = 650.57 (3) Å^3
Mr = 301	Z = 4
Cubic, Fm3m	Dx = 3.073 Mg m^−3
a = 8.6649 (1) Å

Data collection

h = →	l = →
k = →

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Ka	0.25000	0.25000	0.25000	0.0737 (7)*
Kb	0.00000	0.00000	0.00000	0.0972 (12)*
Ga	0.50000	0.50000	0.50000	0.0209 (5)*
F	0.2902 (2)	0.00000	0.00000	0.121

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
F	0.0285 (18)	0.1670 (18)	0.1670 (18)	0.0000	0.0000	0.0000