In a series RLC circuit, the impedance \( Z \) is given by:
\[
Z = \sqrt{R^2 + (X_L - X_C)^2}
\]
Given that \( R = X_L = 2X_C \), we substitute the values and simplify to find the impedance and phase difference. The correct result is \( \sqrt{5} R \) and \( \tan^{-1}(1/2) \).
Final Answer:
\[
\boxed{\sqrt{5} R, \tan^{-1}(1/2)}
\]