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The stoichiometric bulk LuH2 is a paramagnetic metal with high electricalconductivity comparable to simple metals. Here we show that the resistivity ofcold-pressed (CP) LuH2 samples varies sensitively upon modifying the grain sizeor surface conditions via the grinding process, i.e., the CP pellets made ofcommercially purchased LuH2 powder remain metallic but exhibit thousands oftimes higher resistivity, while additional grinding of LuH2 powders in airfurther enhances the resistivity and even results in weakly localizedbehaviors. For these CP samples, interestingly, we can occasionally observeabrupt resistivity drops at high temperatures, which also show dependences onmagnetic fields and electrical current. Measurements of variable-temperatureXRD, magnetic susceptibility, and specific heat exclude the possibilities ofstructural, magnetic, and superconducting transitions for the observedresistivity drops. Instead, we tentatively attribute these above observationsto the presence of insulating layers on the grain surface due to themodification of hydrogen stoichiometry or the pollution by oxygen/nitrogen.Percolation of the metallic grains through the insulating surfaces can explainthe sudden drop in resistivity. The present results thus call for caution inasserting the resistivity drops as superconductivity and invalidate thebackground subtraction in analyzing the resistivity data.

Percolation-induced resistivity drop in cold-pressed LuH2