[(CH3)(3)C5H2NH][ClO4] exhibits rich polymorphism, it undergoes four structural phase transitions: from phases I to II at 356/327 K ( heating/cooling), II-III at 346/326, III-IV at 226 K and IV-V at 182/170 K. [(CH3)(3)C5H2NH][ClO4] reveals a strong optical nonlinearity over a wide temperature region with the SHG efficiency comparable to that of KDP. The piezoelectric properties were studied macroscopically by series resonance method over the phases III, IV and V as well as microscopically by means of piezoelectric force microscopy ( PFM) in ambient conditions. One of the piezoelectric module at phase transition point at 226 K drops to the value twice smaller than in room temperature what up to now was not encountered in literature. Moreover the ferroelastic phase transition IV-V was observed by means of polarizing optical microscope. The choice of the point group of phase I (Pmmm) was confirmed by the SHG measurements. The updated phase diagram is presented.