spontaneous polarization
/ spon-TAY-nee-us POH-luh-ry-ZAY-shun /
Normally a material polarizes only because you push it with an outside field — no push, no polarization. But some crystals manage the trick all by themselves: even with no field anywhere near, they carry a steady separation of positive and negative charge. This unprompted, built-in lopsidedness of charge is spontaneous polarization.
It arises when a crystal's lowest-energy arrangement is itself off-center: the atoms find it more comfortable to sit slightly displaced, so the positive and negative charges never quite line up, leaving a permanent internal dipole. Which direction that displacement chooses is not forced by anything outside — the crystal simply 'falls' one way, much as a pencil balanced on its tip topples in some random direction. Warm the crystal enough and thermal jostling washes the order out, dropping the spontaneous polarization to zero.
This matters because spontaneous polarization is the defining feature behind ferroelectrics, pyroelectrics, and many piezoelectrics. A frequent misconception is that you'd feel this charge on the surface like static cling; usually you don't, because stray ions and electrons from the surroundings drift in and coat the surfaces, neutralizing the outside field. The polarization is genuinely there inside — it just hides itself behind a layer of compensating charge.
Cool the crystal barium titanate below about 120 degrees Celsius and its atoms slip slightly off-center, giving the whole crystal a spontaneous polarization that simply was not there at higher temperature. Nothing pushed it — the crystal chose a direction and locked in as it cooled.
On cooling, barium titanate's atoms shift off-center and a spontaneous polarization appears.
Spontaneous polarization usually can't be detected by stray surface charge, because free ions and electrons settle on the faces and cancel the external field — the polarization persists inside, hidden.