The three polymorphs of RTV whose crystal structures have been published are the metastable monoclinic form I (space group P21, CSD RefCODE YIGPIO) , stable orthorhombic form II (space group P212121, CSD RefCODE YIGPIO01) , and metastable monoclinic form III (space group C2, CSD RefCODE YIGPIO05) . All three crystal structures are non-centrosymmetric and are thus capable of sustaining a SFG signal.