Proton conductivity in a hureaulite-type compound, Mn5 [(PO4)2(PO3(OH))2](HOH)4

The manganese endmember of hureaulite (Mn-hureaulite, Mn-5(2+)(PO4)(2)(PO3(OH))(2)(HOH)(4) with a minor trace of Sr2+) was synthesized under various hydrothermal conditions to yield high-quality crystals up to 1 mm in length. The deuterated samples were investigated using high-resolution neutron powder diffraction (HRNPD) at 12 K and 298 K to understand the proton conductivity in the context of dynamically disordered hydrogen bond networks (DDHBN). Results from Rietveld analysis on HRNPD data combined with bond valence sum (BVS) mismatch landscapes and maximum-entropy methods (MEM) revealed protonic motions confined by two semihelical chains as a DDHBN within 8-membered ring channels running along the crystallographic c axis. A DC bulk conductivity value of 4.5x10(-5) Scm(-1) at 498 K was evaluated from the high-frequency (HF) relaxation mode in impedance spectroscopy (IS) of Mn-hureaulite powder samples. IS spectra acquired at 398-498 K were fitted to two serially connected RC-circuits, equivalent to inter- and intragrain charge transport. A low activation energy of 0.31(3) eV between 398 K and 473 K indicates the ease of site-exchanging protons over DDHBN in the title compound.