I want to thank Tom Bruhns for sending me some corrections to my rather
outdated information about NPO capacitors. First, they're no longer
being made as a sandwich of two ceramic types as I said, but with a
single ceramic. Second, they're now available in quite large values --
Tom points out that AVX has them up to 0.1 uF in 1812 SMT size. Finally,
here's some information Tom sent from an AVX data sheet that gives some
additional quantitative information about the characteristics of these
parts:

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C0G (NP0) is the most popular formulation of the “temperature-
compensating,” EIA Class I ceramic materials. Modern
C0G (NP0) formulations contain neodymium, samarium and
other rare earth oxides.
C0G (NP0) ceramics offer one of the most stable capacitor
dielectrics available. Capacitance change with temperature
is 0 ±30ppm/°C which is less than ±0.3% Δ C from -55°C
to +125°C. Capacitance drift or hysteresis for C0G (NP0)
ceramics is negligible at less than ±0.05% versus up to
±2% for films. Typical capacitance change with life is less
than ±0.1% for C0G (NP0), one-fifth that shown by most
other dielectrics. C0G (NP0) formulations show no aging
characteristics.
The C0G (NP0) formulation usually has a “Q” in excess
of 1000 and shows little capacitance or “Q” changes with
frequency. Their dielectric absorption is typically less than
0.6% which is similar to mica and most films.

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As you can see from the characteristics, modern NPO ceramic capacitors
can be used for very demanding applications, and are often the best choice.

Roy Lewallen, W7EL

Roy Lewallen wrote:

. . .

4. NPO. (Common type is C0G.) These are made from a sandwich of two
temperature compensating types with equal and opposite coefficients.
They have excellent Q and are very stable. They're good for use in
resonant circuits including oscillator tanks, high-Q filters, and other
demanding applications. Q can be very good. A common type is C0G, which
has a temperature coefficient of 0 +/-30 ppm/C.

NPO capacitors are substantially larger than "temperture stable" types,
and much larger than hi-k types. You won't typically find them in large
values.