Abstract
An x‐ray powder diffraction apparatus has been built to perform experiments between 2.6 and 350 K. The experimental procedure and the data analysis allow the measurement of small lattice distortions and very small parameter variations with a good accuracy. Relative variations of the crystal parameters of terbium iron garnet, with an accuracy up to 10−5, have been observed at low temperature. A very small lattice distortion has been determined: TbIG becomes rhombohedral below 100 K; at 4.2 K the rhombohedral angle of the distorted cell is αR=89°52′30, while the cell side is slightly contracted. A noncollinear spin arrangement, an anomalous thermal expansion (Δa=28×10−4 at 4.2 K), and a rhombohedral distortion (β=90°−αR=7′30 at 4.2 K) appear simultaneously at about 45 K. These phenomena are to be connected with a very large magnetostriction (λIII=2400×10−6 at 4.2 K).

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