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Abstract
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-δ 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 Ta and the saturated spontaneous strain (c/a-1)s. The reduction of SrTiO3 led, by contrast, to the decrease of Ta and (c/a-1)s. The relation between Ta and (c/a-1)s of SrTi1-xNbxO3 and SrTiO3-δ coincides with that of Sr1-xLaxTiO3. From the relation, Ta is extrapolated to 61 K at (c/a-1)s = 0. The minimum value of Ta indicates that the conductive strontium titanates having the tolerance factors t ≤ 1 + 1.1 × 10−2 undergo the antiferrodistortive phase transition at Ta ≥ 61 K.