Microstructure and magnetic properties of as-quenched cubic and tetragonal La(Fe1−xSix)13 compounds

•The tetragonal (τ2′) phase with a martensite-like microstructure was found.•The τ1/τ2′ transformation was a kind of chemical order–disorder transformation.•The τ2′ phase had a lower degree of order of Si atoms than that of the τ2 phase.•Magnetic properties of quenched La(Fe,Si)13 alloys were systematically measured. The microstructure and magnetic properties of cubic- and tetragonal-La(Fe1−xSix)13 compounds were systematically investigated. Obtained XRD patterns revealed that a semi-ordered tetragonal-La(Fe1−xSix)13 phase (termed the τ2′ phase in this work) appears due to quenching from the Si-rich concentration range of the cubic-La(Fe1−xSix)13 (τ1) single phase region, whose tetragonality, namely the ratio of short and long axes, is lower than that of the known tetragonal Ce2Ni17Si9-type La(Fe1−xSix)13 (τ2) phase. It was also found that the τ2′ phase has not only periodically configurated twin boundaries as a microstructural feature but also unique concentration dependence of magnetic properties, i.e., Curie temperature and spontaneous magnetization drastically decrease with increasing Si concentration.