Tetravalent metal ions are hydrolyzed under presence of N,N,N-trimethylglycine hydrochloride (betaine hydrochloride, [Hbet]Cl) in aqueous solutions to afford [M₆(μ₃-O)₄(μ₃-OH)₄(μ-bet)₈(κ-bet)₄(H₂O)₄]¹²⁺ (M⁴⁺ = Zr⁴⁺ (1), Hf⁴⁺ (2)) as hydrated perchlorate salts. These compounds were characterized by single crystal X-ray diffraction, elemental analysis and IR spectroscopy. As a result, we have found that fluorite-like [M₆O₈] coordination clusters are formed through octahedral arrangement of six M⁴⁺ linked by μ₃-O atoms. Additionally, each pair of neighboring M⁴⁺ are connected by the μ-bet ligand through a syn-syn bridging coordination of its carboxylate moiety. This interaction seems to prevent further growth of the fluorite structure leading to formation of MO₂. It was difficult to directly distinguish each μ₃-O atom to be μ₃-OH⁻ or μ₃-O²⁻ due to its strongly anisotropic thermal displacement in the obtained structures. Bond valence sum analysis suggested that four μ₃-OH⁻ and four μ₃-O²⁻ are alternately arranged in the [M₆O₈] core motifs. Indeed, such a symmetric structure of [M₆(μ₃-O)₄(μ₃-OH)₄] was confirmed in another phase of 1 at 296 K, where 1 transforms to a monoclinic structure (1′). The number of ClO₄ ⁻ counteranions found in the structure determination is not enough to compensate + 12 charge of [M₆(μ₃-O)₄(μ₃-OH)₄(μ-bet)₈(κ-bet)₄(H₂O)₄]¹²⁺ in any unit cells of 1, 1′ and 2. Instead, large solvent/ion accessible voids have been actually observed in their crystal structures, indicating that the missing ClO₄ ⁻ are located therein and are significantly disordered to make them invisible in the crystallography. DSC analysis revealed that ClO₄ ⁻ and H₂O/H₃O⁺ in the crystal lattice of 1 undergo unique structure relaxation and rearrangements pronounced by cold-crystallization to induce the phase transition from 1 to 1′ with elevating temperature.