Magnetic and structural phase transitions in NiTiF6⋅6H2O

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Abstract
At 300 K, NiTiF6⋅6H2O crystallizes in the trigonal NiSnCl6⋅6H2O structure with one axially distorted [Ni(H2O)6]++ complex per unit cell. This structure is retained on rapid cooling, enabling us to measure χ∥, χ⊥, and Cp between ∼0.05 and 4.2 K in a 3He‐4He dilution refrigerator. χ∥ and χ⊥ are those of a uniaxial ferromagnet with Tc?0.14 K and the easy direction along the trigonal axis. These data, as well as Cp with H∥=300 and 1500 Oe are well described by an Oguchi model in which pairs of exchange‐coupled spins are treated exactly and interactions among pairs are replaced by a mean field. The pair Hamiltonian is CH=D (S2zi+S2zj) −2JSi⋅Sj+gμB(Si+Sj)2. Heff where Si=Sj=1, Heff=H+2J (q−1)M/Ng2μ2B with q=6. We find D/k=−1.83 K, g=2.26 and J/k?+0.012 K. We have also measured Cp for NiTiF6⋅6H2O under equilibrium conditions between 1.7 and 270 K. A first order structural phase transition is observed at 136.5 K with a latent heat of ∼662 cal/mole.