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 P\bar 42(1)m symmetry of the non-modulated phase transforms into an orthorhombic (3+1)-dimensional symmetry, P21212(00γ)00s, whereas the \bar 4 axis becomes the twinning axis.
Keywords: X-ray analysis; borates; incommensurate structures; low temperature; modulated structures; twinning.