Proton mobility in solids. Part 3.—Proton magnetic relaxation in boehmite

Abstract

Proton magnetic resonance and pulse magnetic spectroscopy measurements performed on boehmite in the temperature range from +350°C to –145°C permit two spin populations, P_n and P_w, to be differentiated. The P_w population belongs to the ordered domain of the oxygen-hydroxyl zig-zag chain whilst the P_n population represents either extrinsic lattice defects (below ∼50°C) or extrinsic and intrinsic defects occurring at a higher temperature. In both these defects the longitudinal relaxation rate is attributed to fast proton jumps between H₂O⁺ units and free orbitals of adjacent oxygen atoms: v_D= 1.6 × 10¹⁰ exp (–1800 cal/RT) sec^–1. The transverse relaxation rate is due to the rotation of these H₂O⁺ units (v_R= 10⁷ exp (–935 cal/RT) sec^–1). At room temperature, the P_n population is represented in the n.m.r. absorption spectrum by a narrow band (ΔH² < 0.2 gauss²) whilst the wide band (ΔH²≃ 16.5 gauss²) is due to the P_w spin collection. The relationship between these results and those obtained from previous conductivity and dielectric measurements is discussed.