Li₂CoSiO₄ was prepared in three polymorphic forms. The β_II (<i>Pmn</i>2₁) polymorph was obtained by hydrothermal synthesis (150 °C), and subsequent heat treatments yielded the β_I (<i>Pbn</i>2₁) form (700 °C) and the γ₀ (<i>P</i>2₁<i>/n</i>) form (1100° then quenching from 850 °C). Rietveld refinement of X-ray and neutron powder diffraction patterns reveal considerable Li/Co mixing for β_II, very moderate mixing for β_I, and no mixing for γ₀. ⁷Li MAS NMR spectra have been recorded for the three forms. The mechanism of the Fermi contact interaction that leads to negatively shifted signals is as yet unexplained, but the nature and the number of signals were analyzed in relation to the site occupancies for each compound. The agreement is good for β_II, although the extent of disorder leads to very poorly defined NMR signals; it is reasonable (although not fully quantitative) for β_I, where well-defined NMR signals can be assigned to definite environments; finally, the γ₀ sample surprisingly leads to a single rather broad NMR signal, whereas two well-defined and rather different environments are present in the structure deduced from diffraction.< Leer menos