Fundamentals of piezoelectricity in materials Materials that become electrically polarized when a mechanical stress (compression, tension, or shear) is applied are called piezoelectric. Confirm whether this definition is correct.

Introduction / Context:
Piezoelectricity links mechanics and electricity: certain crystals and engineered ceramics generate electric polarization when stressed and conversely deform when an electric field is applied. This electromechanical coupling is essential in sensors, actuators, ultrasound transducers, and frequency control devices.

Given Data / Assumptions:

• A “mechanical stress” may be compressive, tensile, or shear, applied quasi-statically or dynamically.
• “Becoming polarized” means bound charges shift so that a net electric dipole density (polarization vector) appears.
• Material examples include quartz, Rochelle salt, and lead zirconate titanate (PZT); polymers like PVDF show strong piezoelectric effects as well.

Concept / Approach:
Piezoelectricity requires a crystal structure lacking a center of symmetry. Under stress, relative displacement of positive and negative charge centers induces a macroscopic polarization P. The constitutive relations are commonly written as: D = ε * E + d * T and S = s * T + d^T * E where D is electric flux density, E electric field, T stress, S strain, ε permittivity, s compliance, and d the piezoelectric coefficient matrix.

Step-by-Step Solution:
Identify the property: stress-induced polarization is the defining feature of piezoelectric materials.Check symmetry requirement: non-centrosymmetric structures permit linear coupling between stress and polarization.Confirm bidirectionality: the converse effect (field-induced strain) also exists and is used in actuators.Therefore, the statement that materials which become polarized under applied stress are called piezoelectric is correct.

Verification / Alternative check:
Quartz under shear develops a measurable charge on cut faces. Standard d33 measurements (normal stress–normal polarization) quantify the effect, confirming the definition in metrology practice.

Why Other Options Are Wrong:
Limiting the effect to single crystals or ceramics is incorrect; polymers can be piezoelectric too. Temperature restrictions like “only below 0°C” are unfounded for the definition (though coefficients vary with temperature).

Common Pitfalls:
Confusing piezoelectricity with pyroelectricity (temperature-induced polarization) or ferroelectricity (switchable spontaneous polarization). All ferroelectrics are piezoelectric, but not all piezoelectrics are ferroelectric.

Final Answer:
True