Spontaneous Strains of Conductive Strontium Titanates SrTi1-xNbxO3 and SrTiO3-δ Measured by Synchrotron X-Ray Topography

We measured the spontaneous strain c/a-1 of the antiferrodistortive phase transition in the conductive strontium titanates SrTi1-xNbxO3 with x = 0, 0.01, 0.02 and SrTiO3-delta by means of synchrotron X-ray topography. We found that the substitution of Nb5+ ions into SrTiO3 led to the increase of the phase transition temperature T-a and the saturated spontaneous strain (c/a-1)(s). The reduction of SrTiO3 led, by contrast, to the decrease of T-a and (c/a-1)(s). The relation between T-a and (c/a-1)(s) of SrTi1-xNbxO3 and SrTiO3-delta coincides with that of Sr1-xLaxTiO3. From the relation, T-a is extrapolated to 61K at (c/a-1)(s) = 0. The minimum value of T-a indicates that the conductive strontium titanates having the tolerance factors t <= 1 + 1.1 x 10(-2) undergo the antiferrodistortive phase transition at T-a >= 61K.