DIELECTRIC BEHAVIOR AT THE SECOND-ORDER TRANSITIONS IN CHROMIUM SULPHATE, CHROMIUM NITRATE, AND NICKEL NITRATE

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Abstract
In order to obtain further evidence concerning the nature of the second-order transitions which were discovered by Vasileff and Grayson-Smith in certain salts of the iron group metals, dielectric measurements have been made at temperatures, down to about 100 °K. In the case of Cr2(SO4)3∙18H2O the transition at 195 °K. has a marked effect on the dielectric properties, which is clearly due to the freeing of polar molecular groups, so that they can be oriented by an electric field. The entropy of transition is close to R 1n 4, which suggests that two groups per molecule are set free, to take either of two quantized orientations. The dipole moment of the group which is set free can be estimated from the temperature dependence of the dielectric constant just above the transition point and has been found to be 2.17 × 10−18 e.s.u., approximately equal to the dipole moment of the H2O molecule. It is likely, therefore, that the second-order transition in this salt is caused by the freeing of two of the 18 H2O groups. For chromium and nickel nitrates the transitions have no observable effect on the dielectric properties, and it is suggested that in these salts NO3 groups are set free.

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