The ferroelectric phase transitions of [(1) over bar 01]-, [001]-, and [111]-oriented Pb[Zn1/3Nb2/3]O-3-4.5%PbTiO3 (PZN-4.5%PT) single crystals were investigated as a function of temperature (T between 300 and 450 K), and electric field (E field between 0 and 300 kV/m) by dielectric and x-ray diffraction combined measurements. Under null E field, PZN-4.5%PT exhibits the following phase transitions: cubic (C)-->tetragonal (T)-->rhombohedral (R), during cooling. Under E field applied on a [(1) over bar 01]-oriented single crystal, an intermediate orthorhombic (O) ferroelectric phase is induced at temperatures intermediate between that of the T and R phases. The temperature range of existence of this O phase depends both on the crystal orientation and on the measurement conditions: field cooling (FC), zero-field heating after field cooling (ZFHAFC), or field heating (FH). When E field is applied along [(1) over bar 01], the stability range is within 40 K in FC and only 15 K in ZFHAFC; when E field is applied along [001] or [111], this range is still narrower. The O phase is not even observed in FH for [001] orientation and in FC for [111] orientation. These observations show that the application of an E field favors the transition to an O phase, leading to a single-domain structure, when the direction of the field is parallel to the spontaneous polarization [(1) over bar 01] direction