We are thrilled to see yet another remarkable article on crystal growth of nickelates published some days ago by Kirchhoff Institute of Physics scientists from Heidelberg University. The quasi-two-dimensional Ruddlesden–Popper trilayer phase La4Ni3O10 exhibits an exciting mixture of Ni2+ and Ni3+ ions and an unusual metal-to-metal transition, described in another HKZ-related publication from Argonne National Lab 3 years ago (Zhang, J. et al., Nat. Commun. 11, 6003 (2020)). Single crystals of this phase are very challenging to grow due to the requirement of high-pressure oxygen atmosphere and the sensitivity of the phase to oxygen content variations. The authors from Heidelberg report the successful growth of this compound using a HKZ-150 furnace with 20 bar oxygen pressure. Additionally, another crystal boule was grown and subjected to an innovative post-annealing procedure carried out in the HKZ furnace, where the sample was hold under high oxygen pressure below melting temperature. Different growth and post-annealing processes result in two distinct structure phases, for which the metal-to-metal transition temperatures could be determined accurately.

 High-pressure crystal growth of nickelates La4Ni3O10