Negative thermal expansion in Y₂(WO₄)₃

We present the results of neutron powder diffraction experiments with the D2B instrument at Institut Laue Langevin on Al-2(WO4)(3) and Y-2(WO4)(3) over the temperature range 20 to 800 degrees C. The coefficients of thermal expansion are determined to be alpha(a) = -1.31 x 10(-6)degrees C-1, alpha(b) = 5.94 x 10(-6)degrees C-1, and alpha(c) = -9.94 x 10(-6)degrees C-1 for Al-2(WO4)(3) and alpha(a) = -10.35 x 10(-6)degrees C-1, alpha(b) = -3.06 x 10(-6)degrees C-1, and alpha(c) = -7.62 x 10(-6)degrees C-1 for Y-2(WO4)(3). As in the case of Lu-2(WO4)(3), the expansivity is unusual in the case of Y-2(WO4)(3), as the negative thermal expansion is present along all three axes, (C) 2000 Academic Press.