• borates;
  • X-ray analysis;
  • incommensurate structures;
  • low temperature;
  • modulated structures;
  • twinning

A second-order phase transition with both displacive and disorder mechanisms was discovered in Lu4AlCu2B9O23 using single-crystal X-ray diffraction techniques by cooling down the sample to 110 K. Low-temperature structure modulations are mainly associated with Cu atoms surrounded by O atoms. The fivefold asymmetric environment leads to a special copper position splitting into a pair of general ones so that four O atoms coordinate each of them. Each copper site is 50% occupied at room temperature, but at lower temperature statistical disorder gives occupation and displacive modulations with a wavevector of q = 0.132c*. Tetragonal inline image symmetry of the non-modulated phase transforms into an orthorhombic (3+1)-dimensional symmetry, P21212(00γ)00s, whereas the inline image axis becomes the twinning axis.