Piezoelectric device and liquid ejection head

A piezoelectric device comprising:

• a vibrating plate coupled to a first substrate and configured to partition a space together with the first substrate;
• and a piezoelectric element in which a first electrode, a piezoelectric layer, and a second electrode are laminated in this order along a first direction from the vibrating plate toward the first substrate, the piezoelectric element being disposed on a side of the vibrating plate opposite to the space, wherein when a region in which the space and the vibrating plate overlap each other when viewed in the first direction is defined as an overlapping region, and a region in which a partition wall of the first substrate that partitions the space and the vibrating plate overlap each other when viewed in the first direction is defined as a non-overlapping region, an arm region, which is a region in which the piezoelectric layer is not provided when viewed in the first direction, is located from a part of the overlapping region to a part of the non-overlapping region, and a first stress relieving portion is provided along a second direction intersecting a longitudinal direction of the arm region in the arm region.