Structural investigation of the negative-thermal-expansion material ZrW₂O₈

High-resolution powder diffraction data have been recorded on cubic ZrW₂O₈ [a = 9.18000 (3) Å at 2 K] at 260 temperatures from 2 to 520 K in 2 K steps. These data have confirmed that α-ZrW₂O₈ has a negative coefficient of thermal expansion, α = −9.07 × 10⁻⁶ K⁻¹ (2–350 K). A `parametric' approach to Rietveld refinement is adopted and it is demonstrated that a full anisotropic refinement can be performed at each temperature, despite using a data collection time of only 5 min. Examination of the resulting structural parameters suggests that the origin of the contraction with increasing temperature can be traced straightforwardly to the rigid-body transverse librations of bridging O atoms. α-ZrW₂O₈ undergoes a phase transition from P2₁3 to Pa3 at 448 K that is associated with the onset of considerable oxygen mobility. The phase transition can be described in terms of a simple cubic three-dimensional Ising model. Unusual kinetics are associated with this phase transition. Hysteresis in the cell parameter through the phase transition is the opposite of that normally observed.

Computing details

(global)

Crystal data

O8W2Zr	a = 9.179989 Å
Mr = 586.92	V = 773.60 Å^3
Cubic, P213	Z = 4
Hall symbol: P 2ac 2ab 3	Neutron radiation

Data collection

h = →	l = →
k = →

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

	x	y	z	Biso*/Beq
Zr1	0.0013 (1)	0.0013 (1)	0.0013 (1)
W1	0.3405 (1)	0.3405 (1)	0.3405 (1)
W2	0.59982 (7)	0.59982 (7)	0.59982 (7)
O1	0.20631 (7)	0.4392 (1)	0.44700 (9)
O2	0.78662 (7)	0.5676 (1)	0.55598 (9)
O3	0.49177 (7)	0.49177 (7)	0.49177 (7)
O4	0.23318 (6)	0.23318 (6)	0.23318 (6)

Geometric parameters (Å, º)

Zr1—O1	2.045 (1)	W2—O2	1.786 (1)
Zr1—O2	2.106 (1)	W2—O3	1.718 (1)
W1—O1	1.815 (1)	W1—O3	2.405 (1)
W1—O4	1.707 (1)
O1—Zr1—O1	91.21 (5)	O1—W1—O4	101.96 (6)
O1—Zr1—O2	89.14 (5)	O2—W2—O2	109.76 (5)
O1—Zr1—O2	87.59 (5)	O2—W2—O3	109.18 (5)
O2—Zr1—O2	92.06 (5)	Zr—O1—W1	153.73 (6)
O1—W1—O1	115.82 (6)	Zr—O2—W2	172.53 (6)