Research on rare earth (RE) borohydrides is increasing due to their potential as possible hydrogen storage materials and as solid-state Li-ion conductors for battery applications. In this work, we report the formation of a new bimetallic RE borohydride, KEr(BH4)(4), via mechanochemical reaction between KBH4 and Er(BH4)(3) in a molar ratio of 1:1. Post-annealing of the ball-milled mixture at 175 degrees C for 12 h under 40 bar of hydrogen resulted in an almost phase pure material. KEr(BH4)(4) is isostructural to NaSc(BH4)(4) with lattice parameters of a = 8.4472(8) angstrom, b = 12.4292(11) angstrom, c = 9.6252(9) angstrom and space group Cmcm. An amorphization of the new bimetallic phase was observed at similar to 170 degrees C under 1 bar argon via temperature programmed photographic analysis, which is in agreement with in situ synchrotron radiation powder X-ray diffraction (SR-PXD) as well as thermogravimetric and differential scanning calorimetry (TG-DSC). The in situ SR-PXD data did not show decomposition into the starting monometallic borohydrides. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.