The different translational symmetry of the cuprite crystal sublattices is taken into account by unfolding the phonon spectrum from the Brillouin zone of the crystal into the Brillouin zones of the anion and metal. Such a representation of the spectrum is reasonable because 6 optical branches are essentially caused by the vibrations of anions and 12 acoustic branches are due to the vibrations of metal owing to the great value difference between their masses. The volumes of the Brillouin zones of the anion and metal sublattices are two and four times, respectively, as large as those of the crystal resulting in the qualitative differences in the corresponding unfolded spectra because of the metal tetrahedron vibrations of the distortion type in the acoustic part of the spectrum. These vibrations located on the Х′(1,0,0)–W′(1,0,1/2) and Х′(0,1,0)– W′(0,1,1/2) lines of the metal Brillouin zone are responsible for the negative thermal expansion. The unfolded spectra have only three branches in the corresponding Brillouin zones and this simplifies the selection rules for the physical processes with the participation of the sublattice particle vibrations.

通过将声子光谱从晶体的布里渊区展开到阴离子和金属的布里渊区中，可以考虑到铜盐晶体亚晶格的不同平移对称性。频谱的这种表示是合理的，因为6个光学分支基本上是由阴离子的​​振动引起的，而12个声学分支是由于金属的振动而引起的，这是由于它们之间的质量差很大。阴离子和金属亚晶格的布里渊区的体积分别是晶体的两倍和四倍，这是由于在声学中畸变类型的金属四面体振动导致相应的展开光谱在质上存在差异。频谱的一部分。这些振动位于Х'（1,0,0）–W'（1,0,1 / 2）和Х'（0,1，0）–金属布里渊区的W'（0,1,1 / 2）线是负热膨胀的原因。展开的光谱在相应的布里渊区中只有三个分支，这简化了物理过程在亚晶格粒子振动参与下的选择规则。